There is not much difference other than in their size; the croaking gourami (Trichopsis vitatta) attains around 3 inches, with the pygmy sparkling (Trichopsis pumila) much smaller at 1 inch. A group of whichever is a very good idea, as both will show some interesting interactive behaviours if there are males and females in six or more (difficult to tell gender but a group of six plus should work out OK).
You're correct on the floating plant; Water Sprite is the ideal floater here, as it is substantial. Get this (or similar floating plants) growing before adding the gourami as they will be much less stressed from the lower fish if they have cover. Otherwise, it might finish them off quickly.
When kept in a group, the males will regularly "battle" but not with fights; instead they display in pairs, flexing their bodies to create waves in the water that presumably are detected by the other fish as a sign of strength. During these displays as well as during courtship, the males make grating or croaking sounds like a rapid series of clicks. [This applies to both species.] In a 1992 study, Friedrich Ladich et.al. determined that this sound is important not only in breeding displays but also in antagonistic displays of males to establish dominance. The "croak" is a series of double pulses generated by rapid beating of the pectoral fins. Another study by Ladich in 1998 established that the sounds vary according to the individual fish and the sound frequency had a role in determining the outcome of the encounter between rival fish.