DMC4 is very fun, actually. The combat system is great, and I liked Nero's mechanics. The Devil Bringer is pretty fun to use.
I say you play both, actually, and switch off if you get bored of one. They're so vastly different in gameplay styles that it should keep you entertained. Though if you're picking just one I do still suggest AC1, if only to get you to the awesome that is AC2 faster.
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Gamertag-Murphiroth
Pokemon Black-1978-9490-1161