We find that a lot of people struggle with the choice between the two so there are a few things you should know:
1) RAPS usually ends up cheaper than AFF but can be more expensive if you're slow to progress. On average RAPS costs £900 to qualify and AFF costs £1700 to qualify. After you're qualified jumping costs around £20/jump.
2) They both teach the same things (the aim of both is to teach you to deploy your own parachute at the right altitude and in a stable position). The difference is that AFF receives air to air coaching from 2 coaches. RAPS is more of a pay-as-you-go progression system.
3) AFF is skydivng + parachuting from the first jump whereas RAPS is parachuting until you have cleared the first few levels (around 7-12 jumps normally).
5) There is a high dropout rate for RAPS because although parachuting alone is fun - it's not skydiving... you'll need to be patient for 7-12ish jumps!