I have Tundra, but not Ark 2. A lot of what you get with Tundra is not really possible using other libraries -- it is pretty unique in the sample world. With Ark 2 you get something that sets a certain mood, but its not like you can't reproduce the same with other libraries. Ark 2 has some unique brass instruments that have some good dynamics, but the rest of the library really didn't interest me beyond what I already have, so I didn't buy it. That's just for me though... if you don't already have other libraries that cover similar content, then it may be worth it.
In terms of crossover between the two, I would say there isn't much. I think they each had a different target in mind when creating them, and very different content, so the idea that the two are similar is certainly not the case, IMHO.
So I'd say the opposite to Vastman... between the two I'd prefer Tundra. I don't own Ark 2 though, so take as a grain of salt.