Wow, a question on this board that hasn't been asked before. Good one, too.
Both were products of their times, and Dons. Luca's physical bulk and reputation (the novel tells us, "he can do a job of murder all by himself") were what Vito needed when he was on his way up, and to be a deterrent after he was established as NYC's top Don.

Michael wasn't a traditional Don in Nevada. Buscetta was, by all appearances, a bodyguard--a hired gun for the specific pur. pose of guarding Michael during his trips to NYC, Miami and Havana. His specific purpose, in addition to bodyguarding Michael, was to kill Roth and Ola in Havana. But, I also think he had another purpose: to send a message to Roth that Michael hadn't ruled out Rocco or Neri in the Tahoe shooting, which was why he brought Buscetta, and not them, on his trip. He wanted Roth "relaxed," believing that Michael still hadn't figured out who was behind the Tahoe shooting.

As for personal characteristics: I'm sure they could both shoot straight. Luca looked like he had more muscle, but Buscetta had an air of quiet deadliness.