I went with the 12.9" model, chiefly for drawing, writing, some work, and making electronic music (Sunvox, Caustic). I also still have me iPad Air 2, which I use for media consumption and browsing in bed, on the couch, in the kitchen, etc. I don't really take either of them outside much.
If you plan to take yours to college, the 10.5" may be more suitable as it's less bulky, but the larger model is very suitable for the other interests you listed (drawing, making music, even writing). It may depend in how much other stuff you lug around usually. If you just drop it all in a bag, the larger iPad doesn't make such a huge difference, I suppose.
As for the keyboard, I'm still undecided between the ASK and the Apple Magic keyboard. The latter is almost half the price, has full size keys, and can be used for other devices also. But for school or college, the ASK may be better because it also serves as a cover. Worth mentioning that the virtual keyboard of the 12.9" model has full size keys. It's slower to type on due to the lack of tactile feedback, but it's actually not so bad. I find it rather usable.
You probably can't go wrong with either of them. If you can, try out both, see if one of them appeals. If you have a large phone already that you use a lot, the bigger iPad may complement it better, also.