Although both Regnar and Jayce can burst an adc down very quickly, being a rengar main, his play-style is solely designed around bursting their squishies, assassining, and picking people off. His burst is also far greater than Jayce's. Although I'm not a Jayce main, i know that Jayce's burst is considerably less, and his power comes from his mid game poke, and his utility. You're not going to be setting up plays on your own as Jayce like you would with Rengar, and you're not going to be in many situations where you would have a chance to burst the enemy adc, so definitely Regnar imo.
As for ADCs I've only really played 3, cait is the easiest, vayne is the most mechanically demanding, and ezreal is the funnest to play, and is prob one of the few adcs that can go off and do their own thing, due to his mobility and kiting potential. Lucian Corki and Trist are also A tier at the moment.