both are strong atm, if u want a perfect example of how mental kha'zix is if he gets even slightly ahead check out last weeks LCS Match between TSM and i think it was CLG with Regi as Kha'zix just absolutely steam rolled through the entire team, but jayce has amazing poke and utility to assist with his speed buff, its down to personal choice i guess, i didnt like kha'zix when i bought him a couple of weeks back but im more of a supp player