Quote Originally Posted by R_Ravenclaw
I think they were alike in basic ways (i.e. arrogance and humour), but they were different in so many little ways. I think James knew what he wanted from life. He was a spoiled kid, and he had a good life. So one day he met this girl and knew that she was the one (not to sound cheesy or anything ). In that way, I think James knew his mind well. But Sirius obviously dated around a lot. He didn't even seem to be really looking for the right girl. I see Sirius as a bit more reckless. I don't think James would've lost his temper (or however the prank came about) and told Snape how to get into the tunnel, where he knew full well that there was a werewolf.
I think that Sirius didn't really know what he wanted either. He had been living with his family that he couldnt' agree with and only knew he didn't want to become them. He hadn't yet figured out what else he valued or wanted. James knew his mind well because he had grown up agreeing with his family and could figure out what he valued in life rather than just know he agreed with his family.

Sirius was more reckless because he knew his family wouldn't approve of it. He hated his family so much that he would actually do whatever they didn't approve of.