The Croll is (has been) slightly larger. It also has a twist in the top and bottom attachment points which allow it to sit flush against your chest when used as a middle attachment in ascent systems.
I use a Croll for a knee ascender in a ropewalker system. You could use a basic the same way. As I see it, the Croll will do everything the Basic will, but NOT vice-a-versa. Overall the Croll, IMO, is more versatile and I do not see a negative trade off. Anyone else?