Choosing the right backpack

Hmm, I take it that by "spine-friendly" you mean to say your actual backbone..?^_^

Anyway, I wouldnt recommend those "trolley bags" as you described. It would be very impractical and too much of a hassle extending the arms, bending down to lock the arms up/down, pushing it when the tires are uneven because of use, etc. Most especially if you have only a few books to carry. Though if you do have too many heavy books to bring with you, then this trolley bag is what you need. You can just ask someone to lock/unlock the arms for you, or just lock/unlock them when you're still sitting down.

As for the real backpacks that you slip both arms into, get a slightly padded one on the shoulders, that would be more comfortable. Oh, and I think a bag that has a book support on the bottom can help ease the tension when carrying heavy books. Otherwise a backpack "sack" would do just fine.

Why not go into the department stores and try out those bags that you do like the design/appearance. Maybe an extra pocket on the side would help..? Or something like that. Best way to choose the right one is to try it out first.

Goodluck and have fun!