I would play both if possible, play both the demos and decide which one you think you feel is worth buying first on your own. Luckily since both games have demos(well MGR will very soon) you can try them both.
You really shouldn't let random people on the internet decide what game is best for you, both games have quite a few videos on the net and a ton of info and even demos so use those to decide.
I mean sure I could say get MGR, but I'm a HUGE P* fanboy who has liked every game they've made so far and already has MGR pre-ordered I also HATED DmC's demo so I'm not getting it (maybe the PC version eventually).
But unless you have my EXACT same personality and tastes my opinion shouldn't really mean jacks*** to you. Buy what you like and to hell with everyone else's opinions I say.