I gather your looking at taking SleazyJet? In which case it is almost six of one, half dozen of the other.
IMHO Gatwick does have the edge with the Gatwick Express non-stop from Victoria in half an hour with trains every 15 minutes. You then arrive at the South Terminal.
For Luton, you travel from Kings Cross to Luton Airport Parkway (about 40-50 minutes) and then you take a shuttle bus to the terminal.