I've got enough IP for one of them.

Give me pros/cons for each and who's better in your opinion?

Probably Caitlyn if you're new to AD carries

REason being is that Vayne has short short range, and is more of a cleanup AD carry. Caitlyn has longer range and is probably more newb friendly

Caitlyn

kog maw

Vayne takes more skill and is better late game but Caitlyn has a way better early game.

depends on what do you like

if you like chasing enemies and a better late game, vayne is better for you

if you like trapping the enemies and baiting them as well as a strong early game, go for cait

i like vayne more because i can't chase with cait and i'm not good at placing those traps

also vayne has more dmg out put but 100 less range than cait
cait's range is 650 and vayne's is 550

Kog'Maw is good, Vayne takes skill and has more power, Cait's noob friendly.

Actually, if you're going to have 2 ranged AD carries in a single team comp, Vayne + Cait is the way to go