Genesect is far superior to Meloetta. It is one if not the best scout in the game with Download and U-Turn, but it is also a fearsome special attacker or setup sweeper. It is also nearly impossible to wall or counter with its great movepool and superb typing, the only non Uber Pokemon that stand a chance against it being Heatran. Overall it's terryfying Pokemon.
Meloetta isn't bad but Genesect is just several leagues above it. Sure it have a neat form chance meaning it can counter a lot of different things and keep your foes guessing, but it's the Jack of all trades but the master of none. It's too slow to properly sweep, lacks recovery so it can't stall, lack any great setup to properly set up, and just generally have no roll specific to fullfill. It's not a bad Pokemon, but there are others who can do its job better. When choosing between the two however there is little competition however, Genesect reigns supreme.