I'm going to give a long-winded answer because I have both and love them equally.
Gallery tote - this is light-weight and very roomy. I use it mainly for going to my office at school, so I carry a lot in it (small lunch, wallet, wristlet, phone, magazine, water bottle, small travel mug, etc.). I also use it in place of a diaper bag sometimes. I like that it's got a few outside pockets (although these don't hold a lot - just my keys in the side one, and some lipglosses and kleenex in the end ones). I used it this weekend for a ferry trip, where I knew that I'd need to throw all kinds of odds and ends in it and be able to reach them quickly (my kid's stuffed animal and toque, leftover lunch, snacks, etc.). I couldn't have used the Legacy for that.
Legacy shoulder tote - this is also very roomy but doesn't really fit bulky things like a travel mug, big magazine or book, big water bottle. I use it when I need a true shoulder bag - e.g. one that will sit comfortably on my shoulder without slipping down. I like it for when I'm going to be pushing the stroller a lot, for example. The drawback to this bag is that it's not really comfortable to carry in the hand because it has such a wide strap. I can carry the Gallery tote on my shoulder, in my hand, or in the crook of my arm. However, the Gallery tote doesn't sit as comfortably on my shoulder for long periods.
Really, they're both GREAT bags, just very different. Between the two of them, plus my swingpack and my soft duffle, I've got all my purse needs almost completely covered! Anyway, it really depends on how you think you'll mainly be using the bag. If you want something to use as a work tote, small carry-on, etc. then go with the Gallery tote, but if you want something as a roomy but not bulky everyday shoulder bag, then go with the Legacy.
Oh, here are pics of both of them on my shoulder to give you an idea of size (I'm 5'5" and about 130 lbs).
