Being both a 40K buff and a zombie buff, it's hard to compare the two, as I love them both. However, Dead Island has a number of technical issues, and despite Techland claiming the PC version was developed separately, it's quite clearly a console port - Space Marine never claimed to be anything but, and even being a shameless console port, it runs quite well on PC, and in fact, it surprisingly even controls better, which is odd for a third person action game.
Now that Space Marine is getting co-op, I'd say it actually has more staying power than Dead Island, especially if the cooperative mode is as fun as the competitive. Unlocks and such will carry over between the two, too! Overall, I love both equally, though DI is better for single player/co-op (for now), SM for multiplayer.