The machine is the exact same. I myself ordered the 7lb and wished I had spent the little bit more for the 15lb hopper.
The problem is that in doing any 5 gallon all-grain batch you're using more than 7lbs of grain, since I usually brew alone it's a bit of a challenge to run the drill, hold the crusher over the bucket and pour into the crusher all at once. The one time I had a helper it was no problem whatsoever.
I'd say if money is really tight and/or storage space is limited get the 7lb and deal with the slight inconvenience, if not get the 15lb and you'll be glad you did.