We have had both. The Land's End bag held up really well. My daughter had it from kindergarten through 2nd grade, and honestly the only reason I got her a new one was that we had picked a really pale blue color, and it was nasty and stained after several years of use. Otherwise, it was in great shape. We still use it for outings where we might get grubby (zoo, etc).
I got Pottery Barn Kids ones this time (for my daughter and then my son who started school this year) because they had really cute patterns and I thought the bag would stay cleaner looking if the pattern on it was busy. Also, my son was really into sharks, and they had a shark pattern for the backpack, lunchbox, etc. So far they have been great. They are nice and durable, and have stayed pretty clean looking. My only complaint at all is that one of the buckles on the front of my son's backpack keeps sliding off the strap. It isn't a strap that is particularly functional--we use it to hold his jacket on when he isn't wearing it, and doesn't want to carry it. So it isn't a big deal--it isn't a strap that is necessary to the function of the backpack itself.
If you are also looking at lunch boxes, I definitely like Pottery Barn Kids' better. They have that solid molded interior (which Land's End used to have) and it is much easier to keep clean than most other lunch boxes.
Also, if you call Pottery Barn Kids, and ask them for last year's colors, then you can get some awesome deals. I got my daughter's backpack for $9.99!
P.S. FOrgot to add that I had an LL Bean backpack all through highschool, took it with me to college, and when the zipper broke, I called LL Bean and they replaced it! But that was in the 90's. Not sure if the quality is the same now.