I collect both Lizardmen cause I like dinos and Tyranids because they look sweet.
In terms of how the model range, hmmm... They got the Ymargl Genestealers, which are like Saurus scouts, and regular Genestealers for Saurus. Warriors which are like Kroxigors with cannons. And then all the masses of Gaunts which are like Skinks. Carnifex, Trygon, Mawloc, could be comparable to the Steg and Carnosaur. Rippers are the jungle swarms. Zoanthrope is their Slann. Hive Tyrant is an Oldblood on a Carnosaur w/a wizarding hat??
But in play the Tyranids are more focused on bringing enough cannon fodder to be able to get within assault range, attacking in waves to stop the hail of bullets from hitting their monsters. Very much offensive. I don't really see LM being played like that. I mean, yeah, you throw Skinks under the bus sometimes, but not EVERY time. I actually care about my little lizards, unlike Termagants. I'll gladly put a couple units of those little buggers in harms way.
Fluff-wise I always thought they were closest to Tau with their societal castes and the way they work with each other for the betterment of their race. They are supposedly a young race though, so that's not very parallel. The Kroot look more like them than anything though.