Invulnerability.
Rapid Regeneration, you heal injury, but Invulerability, you don't get injured in the first place. Still likely to feel pain, though, which can be good or bad, if you have that Claire Bennet, "Am I still human?" complex.
Either way, the only true weakness to being invulnerable is to suffocate, drown or starve. Regens can be killed many other ways.
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)