They do look similar. Platies are shorter and rounder. Mollies are longer with what I call a flat shovel head lol. Mollies can grow very big (4 inches long) in a proper sized tank which allows them to and I find them territorial and aggressive towards others. The display tank at LFS has sailfin mollies which are easily 4-5 inches long and very stocky. The males bicker over females just like guppies.
Platies are very much community fish. I haven't had any problems with aggression with them.