Why I use a backpack:
1) If I stop to do some errands to/from work the backpack is much more convenient to take into a store.
2) Hydration bladder
3) I hike a lot so the backpack does double duty.
4) I treat my commute ride as a workout - I ride as hard as I can. So, I'm sweating anyway.
5) It holds everything I need - clothes (jeans, shirt, underclothes), raingear, emergency tools, lunch.
5) I'm very comfortable with it.

Which backpack for me: REI Alpine day pack (about 2300 cubic inches, I think).

Obviously, you have to make your choice based on your own needs and comfort. You'll get many opinions from folks, many swearing that their system is the best. Which it is - for them. YMMV