i don't think whether or not hulk can die should be an issue here....due to both of these characters rapid healing factors and the fact that they both use lots of brute force to get the job done....it seems a more relivent question might be who's physically stronger, and who's more prone to getting knocked out?
Every drop of blood Lobo spills creates another Lobo.
He always knows the location of anyone he's ever met. His inner tracking ability, it's what makes him the best bounty hunter.
Strenth on Par with Superman.
Super healing factor, and cannot die, will always be reborn even if fully disintegrated.
In Our Worlds at War, he took on Darkseids planet.
In JLA/Avengers he shut down the Shiar Empire solo.
I'm going with Lobo.