I think Nate has taken a bit longer to get in his groove. He's continually getting better while Nick I think peaked a bit earlier. Nate is more like a Verdum where his skills increase. I think Nick just was a bit more natural at it and up to his suspension was the better one.
Nick had great fights with Noons, Penn, Inoue, Shamrock, Gomi. His last few fights have been against really tough opponents though, so only time will tell. I see him getting suspended again though honestly. I'm a big fan of the Diaz brothers, but let's be honest here. His constant troubles started over 4 years ago and testing has gotten worse.
Nate has consistently gotten better. He picked up a win over Cerrone, Gomi, Miller, avenged Maynard loss in a BIG way. All his losses have been against good people (with maybe the exception of Thompson). And with LOTS of bonuses in there.
Time will tell for both. I think Nate will get in a position to get a LW title match again (not saying he will win it. I hope Alvarez holds it for YEARS). Nick though, I just don't know. I don't see him getting anywhere past some of the good welterweights right now.