That's a tricky question. I prefer backpacks for the looks and ease of carrying but prefer should bags for functionality. I was in a great dilemma when I was shopping for a bag. didn't know what type of bag to go with. I didn't know what was more important: functionality or carrying. I decided that since I was looking for a specific use bag (photo), I should concentrate on functionality. Ended up getting the Crumpler 7MDH (shoulder bag) and never looked back. Yes, a backpack is easier to carry and more comfortable, but at a moment's notice if you need to pull your equipment, or need to change a lens, a shoulder bag is much easier to work with. It also gives you a more secure feeling in crowded places as not many people (if at all) will attempt to mug you with your bag secured at your side.
On the other hand, you should also weigh your needs based on function. If you like hiking a lot, definitely go with a backpack.