both aren't particularly good...
w/ constantine you are getting a plank of wood lead actor (Keanu)...w/ underworld, you get good looking chick in tight leather...masturbation factor goes to underworld.
Constantine has better symbolism (as little as there is) w/ the night club being purgatory and such...so constantine wins the symbolic award.
Soo it all comes down to if you wanna rub one out, or watch a movie that might tickle you intellectually...assuming your intellectual level is that of a 8 year old.