I'd say don't buy anything new/fancy just yet. At most schools, you start getting some nifty little swag right off the bat - our first day, we all had nice backpacks courtesy of Hill's waiting for us. I don't know if all schools get a backpack, but I think many do. It's a pretty sturdy pack with padded laptop sleeve and probably 80% of our students carry those around. I still use my Northface Recon backpack just cause I'm more comfortable with it and it probably holds a tad more, but the Hill's one would have been fine for me, too, I just keep it as a backup backback.
I definitely prefer the backpack over a shoulder bag, which I used for a lot of undergrad. I try to keep my load pretty light, but I pretty much always have to have my laptop, then you put like even just two books in there and it's way more heavy than I would want hanging off one shoulder. I'm sure I'll be crippled enough by the time I get through vet school, so if I can at least keep my heavy load centered on my back I might be a little better off!