Well, both are Don Bluth films, both star mice and both are utterly amazing
For starters, I absolutely love the animation in these two films. The style is incredibly well detailed, to the point that some folk complain it's a bit over-animated. I can see where they're coming from but for me that's part of the charm. The movements and gestures of the characters are so energetic and fluid, and the backgrounds are phenomenal in their detail. it's quintessential 2D animation at its finest.
Of course this wouldn't mean too much without a good story and cast of characters, but both films are excellent in that regard. Both stories are incredibly heartfelt and touching, handled with just the right amount of sentimentality. The leads are very likable, Mrs Brisby for her modest yet determined personality and Fievel for being just so damn cute. Only thing I would say is I'm not entirely convinced about the comic relief characters, but they're not too distracting and they have their charm.
More than anything though, these movies have some very dark themes considering their U ratings. The animal lab scenes in Nimh or the subtle way American Tail challenges the reality of the American dream in particular stick out, it's pretty bold film making on the studio's part, and it's hard not to admire them for that reason.
I think I'll track down The Land Before Time next. It's a shame I didn't grow up with these movies (Though I can't help but feel I have seen LBT, I vaguely remember having fond feelings for it), I imagine they're wonderful films to see growing up. Great stuff nonetheless