Different spirits work for different purposes and so I work with both depending on the situation. Since most of my practical work is using GoM books for their time efficiency and relative effectiveness, I find myself turning the most to the angelic workings in their books. Working with demons is more detailed and time consuming as far GoM goes. But I can go into a lot more detail on this:
Angels
Angelic workings can be just as complicated as demonic workings if you opt out of GoM and into more traditional methods such as full blown evocations or the Trithemius crystal workings. In the traditional cases, you would still need to have extensive preparations, purifications, and workings in order to get to the full result of the working. This is why I prefer the GoM angelic workings because they are quick, streamlined, and effective. They also do not require much in the way of offerings to be prepared which can be helpful for people are time constrained like me.
Of course, we should also discuss about when we should consider using angels. I mentioned that different angels and demons can be used for different purposes, which is what I will focus on here. The common thinking is that angels are great for workings that are broad and salient in nature as well as great for inner work. So when it comes to inner work, we could mean things like personality or character changes. And for broad and salient, we could refer to longer term goals in the mundane world or more profound spiritual goals. In the GoM works, angelic magick is used for both mundane and inner work results. So I have used angelic magick personally for more than broad goals or inner work. But I find them to be very effective for inner work particularly.
As to the nature of angels, you either use your own experiences to inform you or you use occult theory to inform you. I am not clairvoyant and have limited astral senses, so I have limited direct communications with them. But based on the energy I felt when working with them, they are more subtle and quiet than demons. Their results also tend to come more quietly than my workings with demons, with a couple of exceptions. So in this sense, my experience says that they are more subtle and quiet in how they manifest things for most requests.
From the theoretical perspective, I will use Kabbalistic theory to really focus on the nature of angels. With demons, we usually regard them as ancient spirits that have been either appropriated from other pagan beliefs, but in being these spirits they are also more individualistic than angels. In certain Kabbalistic works such as Brit Menuchah, angels are referred to as vessels of light that receive the divine light of god and are bound to direct that light in certain ways that power the manifestation of reality. They are subject to rules by divinity into what ways they channel the divine light and therefore have specific purposes they are bound to by God / the creator. You could view them more as machines in this way and so they execute their instructions according to the rules they have to follow. Others have experienced angels as being more individualistic than this, but if you want a theoretical understanding of this, then this is a valid one to use.
Whether or not they are truly just machines of light with no free will is not important in my eyes. They are legitimate spirits to cooperate with and work with. They have brought me great results, protection, and understanding through my exposure to them and their capabilities. In the end, if you choose to work with angels, you work with them according to either your own rituals or according to the rules of the ritual you are using from another book. If you follow the rules, do the rituals, and aim for a result that is not impossible, then angelic magick will work along the lines I described or even in a completely different way.
Demons
Demonic workings are usually more lengthy and require more preparation to do than angelic workings, both for GoM workings and traditional methods. I understand that there are also new pathworking books out now for this which are much simpler, but personally they never really appealed to me. But from a technical standpoint, you have to account for elements such as protection, divine authority, and offerings which are not always requirements for angelic workings. Of course some of these things are only required if you treat demons as "demons" rather than spirits and if you are approaching from a more Solomonic perspective. If you are working either from demonolatry or treating the demon as the spirit they originally were, then you would use other belief systems to inform your working.
The nature of demons are much more individualistic than angels as mentioned previously. One demon is likely to be different from another spirit because they were adopted from completely different beliefs systems prior to their inclusion into Abrahamic religions. Though they also adopt an aspect of their "hell demon" character when they are called as such, their hell demon aspect is still linked to their original aspect and this is what makes them so full of "character". In this sense, they are very different from angels who are more like automatons that do their functions in the manifest world. But ultimately this is based on theoretical conjectures; it is your own personal experience working with angels and demons that determines what you think about their natures.
In terms of results, I find demons to be very effective for mundane results. Some of my greatest mundane results in dating / seduction, money, and just overall visible changes came from my workings with demons. The theory is that since they are "Cthonic" spirits that are closer to us in vibration than angels, they are able to affect the physical world more easily than angels. Maybe to a certain extent, this can be true based on how I have experienced my results. But I cannot say objectively if this will be the experience for other people. I just know that if I really want or need something to happen quickly and powerfully, I will use a demonic working to get the job done as this has always been an extremely powerful way to manifest things for me. I combine these workings with adequate security measures and also offerings which provide a way to not only do things safely, but also to tune the effectiveness of the ritual. While taking more time to prepare in general, offerings are a great way to empower the demonic working. Angelic magick usually requires less on the offerings side and one can even use gratitude itself as a form of offering to the angel. But with demonic work, it is usually encouraged to use offerings for powering the manifestation of the work and as a form of thanks. If you use blood, for instance, you can even combine the advantages (and disadvantages) of blood magick into your working, which can be even more powerful. From this perspective, working with demons requires more creativity, but also gives more flexibility.
I find myself more drawn to demonic workings and if I had more time to work with them, I would even use those rituals more frequently. I experience more "feeling" when I do a demonic working and I think their energy is thicker than an angel's energy. I would not necessarily say the energy is darker, but you definitely feel more voltage. It is denser and this corresponds to their cthonic nature more. And they correspond to my own nature more than angels because I am also an individualistic person myself.
Overall, these are my experiences, but remember that you will find your own experiences to vary. If you want to truly know the differences between these 2 classes of spirits, then you need to pick the correct rituals and do the work. Then you will understand if what I say, or anyone else says, is true.