I don't think the complexity of developing for one machine or the other has much to do with it. Of course doing the same effect on both machines it's most often a lot trickier on a C64 compared to the Amiga. However, just because of that there are different standards of what is acceptable to release are on the machines and I do think that the state-of-the-art demos on each machine requires about the same amount of time and talent to produce.
Possibly one thing that the C64 has as an advantage compared to the Amiga is the social structure of the scene. The C64 groups have for a number of years done thing separately from the remainder of the scene, this way they have managed to build a very nice environment and social structure. Us Amiga sceners have been trying to be a part of the scene in general and now find ourselves marginalised to quite a large extent.