DeRozan
It's close and they have different styles, as many posters have noted. Both guys are on the rise and are killing the preseason. Both will have big years, health permitting.
Klay came in the league as strictly a shooter and has become much better driving, in the midrange and working in the post. He could crack 20ppg this season. He's the #2 scorer on his team, but DeRozan would be #2 to Curry as well. His ballhandling and shot creation skills are up to an acceptable level but not outstanding. He's an above average passer playing on a team stocked with elite passers. More an "in the flow" scorer than an "attack" scorer.
DeRozan has gone from literally the worst 3-point shooting guard in the league to a slightly below average one over the last 5 years. He has gotten noticeably better every year except for Dwane Casey's first year in Toronto when he changed his whole game and actually started playing defense. This is DeMar's first training camp coming in as the #1 scoring option.
His midrange game is elite, his driving and post game is well above average if not elite as well. His big challenge now is to speed up his reaction time to double teams and to improve his ball security. He makes excellent use of fakes, feints and pivots to create space for shots and drives, now he needs to extend that to passes. He's an average passer on a team where only the point guards are above average passers and everyone else is OK-to-bad. He makes tough shots, draws fouls at an elite rate and is more of an "attack" scorer than "in the flow" like Klay.
Some numbers to back up DeRozan as the better scorer:
Klay had 7 games last season scoring in single digits while playing over 20 mins
DeMar had 2, one of which was the season finale where he basically sat the 2nd half to rest for the playoffs.
Klay's career high in points is 38, DeMar's is 40. Very close.
In the 2014 playoffs, DeMar scored just under 24ppg on 38% shooting. Klay scored 16ppg on 41% shooting. Klay also had 2 single digit scoring games in that 7-game series. DeMar had 0.
Both guys are still on the upswing so Klay could pass DeRozan if he develops his skills better but as of today he is slightly behind.