I've been thinking about this because I like them both and I'm starting to play more ranked games.

Draven is pretty OP IMO but that cait poke. it's really whoever you like better and you can always get the other one later.

Caitlin is an extremely solid choice for an ADC. Her devastating range leads to a super strong laning phase, and she has the best tower siege potential out of all the ADC with a mixture of great range and traps. She will always be a great pick regardless of your skill or elo bracket.

I would't recommend purchasing Draven unless you truly fall in love with the champion. He can be a blast to play with his insane burst damage and outrageous character. He is no where near as user friendly as the rest of the adc cast though, and is rarely a prefered pick over Ezreal, Graves, or Caitlyn.