I wouldn't bother with the 38cc unless you can get one for a lot cheaper than $99. You can pick up a 42cc or 46cc for a little over $100, and the weight difference is negligible. If it is for a 1/4 scale Cub though, the Poulan 33cc - 38cc would be good. Obviously overpowered, but not outragiously so. The bigger Poulans are way too much. I wouldn't put my 46cc on my 1/4 Cubs - that would be insane.

Do a search for Poulan on these forums, there are plenty of talk on this subject.