Heres my opinion as a Diamond 3 ADC:
Gtaves:
Learning curve: Easy
Pros: -High Burst with his Q-R combo
-His smokescreen is a very strong utility tool for removing enemy vision
-Graves is one of the top meta picks right now since AD casters are currently the top ADCs
Cons: -525 atk range
-Late game, his burst Q-R combo dmg falls off so his main source of dps becomes his autoattacks
Draven:
Learning Curve: Medium
Pros: -Has a very high snowball potential due to passive
-He has a mix of moderate burst and high sustain dmg if you can keep up his spinning axes
-Hes a big lane bully
Cons: -If you can't catch axes, his dps heavily drops.
I personally prefer draven because i do think that he can easily come back from a gold deficit at any time in the game and graves' dmg easily falls off when he gets behind.