Weight doesn't bother me, so I'm the worst to ask that question! However, I will say that I looked at both satchels because I thought of buying one (or both). Hands down the '06 is nicer (leather), but I didn't care for the small size and the flap bothers me (hard to open). So I bought a Mandy instead. IN fact, I'm not the only one that did that in the outlet at the time, and the SA made the same recommendation.
As for the '07 satchel, TBH, I don't like the look as well as the '06 (so I didn't buy it either), but I also don't agree that it was THAT much lighter than the '06. I think the leather is thinner on the newer Legacy pieces, and I have no doubt of that when I compare something like the Ali with the new shoulder flap. But, honestly, I don't know that it's going to equate to a major change in weight...or enough to help you if this already bothers your back significantly on the '06. Of course it's partly what you pack in a bag, but the Legacy leather bags are all heavy. Everyone that picks up my Bridgit always comments on how "heavy" it is, but it's soooo light in comparison to everything else I have. It's all relative... If you want a light bag, then I don't know that Legacy bags are your best option.
I think I would recommend trying that bag on if you can...or something similar before purchasing. Also, one thing I do when a buyer asks me this question is to physically weigh the bag for them free of all contents. That way they can compare it to one they already own at home. Of course sometimes bags aren't weighted well (evenly distributed), or the straps/handles make it such that it feels worse because it pulls on the shoulder. That's another thing you might consider...would a single strap bag that's a similar weight be better than a satchel type??
Both are beautiful bags, though! I'm personally a fan of turnlocks over the buckles, but I do love the look of the '06 satchel overall better than the '07.