Impossible to answer unless we get more info. Are you going to buy it on the internet or buy locally? If you are in the US (I guess that you are since you say nothing about it) it is of no help for you if I tell you what I would buy locally where I live. Also backpacks is like shoes. What fits me could be wrong for you.
Personally I would buy panniers (front and back if needed) before buying a backpack for bike use. Only thing I would maybe carry on my back when on a bike is a laptop or eggs.
I sometimes use a light backpack when riding a foldingbike in a city or similar when I fold the bike several times in a day and/or go in and out of shops a lot.
Edit: read trough the sticky "Post your utility bike thread
" on the top of this page to see how others carry groceries and stuff on a bike.
Last edited by badmother; 01-09-12 at 05:19 PM.