Barring some trade for a veteran high quality wing defender (Battier, Artest, etc.) then I have to believe it's Nicolas.
You don't get many 19-20 year old wing defenders as polished as Nicolas already is in the NBA. Think about this, he's still raw as hell and already is a far more valuable complimentary player than Martell ever was in his first 3 years: better passer, better understanding of spacing, better handle, more length, more quickness and athleticism. All he lacks is a consistent jumper and some strength, two things I expect will develop nicely over the next couple of years.
Maybe he'll be a changed man after sitting out for a year and being just a little bit older and that much more mature, but one of my biggest complaints about Martell is that his confidence has always seemed to rattle so easily (or maybe it's just his focus?). He's talented in his own way, but he's never struck me as someone who has that self-contained grit and determination you look for in a hard-core competitor; his body language has never said 'winner' to me.