[Batfish]

Although both are tops in competitive racing, I would choose the Associated for just one reason: the diff holds up better to power. I've had enough experience with Losi diffs and race with enough Losi drivers to know that the Losi diff just doesn't hold up as well.
All that really means, however, is that you need to re-build the Losi diff more often. If you like doing maintenance, then either is a great buggy.
If either has better support at your local shop/track, that should also be a consideration.