If Nin is more fun for you than Regen then you should go Nin over Regen. Both are very good sets, numerically.
Personally, I'd go for Ninjutsu. My usual play style benefits a lot more from +DEF than +REG, and I like having the maximum chance not to have my Hide/Placate broken by some random mob.
"Prediction is extremely difficult. Especially about the future." - Niels Bohr
I went Elec/Nin. Absolutely love it! It's a beast when you get it soft capped with IO's. I can't tell you how many times I have taken over as lead Brute on a team..........oh wait, I'm a Stalker! <slaps forehead> hehe.
I'm torn between Ninjitsu and Regen; Regen for thematic purposes, Ninjitsu for the fact that Ninjitsu is fun as hell. Any thoughts or opinions?