I haven't played MG:R yet. I've read opinions and it seems fans are somewhat disappointed in this game, and that it has issues.
I played and enjoyed DmC for what is is, it's a really fun action game but it's not really a true to form DMC game.
Anyway, I was thinking about getting Vergil's Downfall DLC, fan impressions seems that it's really good, Vergil's story is interesting and more true to his DMC3 persona, and the platforming crap is faster since Vergil can teleport around. Combat looks solid: http://www.youtube.com/watch?v=JFv0ktUKZ-Y
But I really do want to play MG:R. Should I buy Vergil's DLC for 8 bucks and rent MG:R. I heard it was pretty short? That would be financially reasonable for me. Thanks.
This really comes down to what kind of gamer you are and what kind of experience you want, cause these games play completely differently and excel in different areas. Neither of these games are perfect, so it's going to come down to your personal preference.