They are literally in the opposite side of the spectrum. Penn loves to fight and roams Hawaii looking for trouble. He would fight 50 more times if he could. Nick hates to fight, he didn't want to come back, he obviously did it because he has money trouble, he didn't want to let his hands go against Robbie (although we saw glimpses of that) and when he ate the first good punch he just quit.
And it's not like Nick has some huge legacy. He had one good run in Strikeforce's worst division and then got manhandled by GSP. BJ is one of the original GOATs and for some he has an argument for LW GOAT even today.