I personally think that mollies are hardier and easier to keep alive for a beginer, doesn't get its fins nipped, doesn't have huge fines to worry about all the fin diseases. But they also are bigger, so you can't get as much, not as colorful, and not as fun to breed. But teh guppy has teh colors, the looks, the easinedd and willingness to breed, has babaies more often sometimes, and they are small so you can a bunch, you can make your 'own' color, shape, size fish with the guppy.
Ron