Personally I liked red Dead Redemption, but I loved New Vegas. So I would say New vegas, but that's just me.
For me New Vegas was a great game because it expanded the Fallout universe greatly and offered some really great stuff. The game allows you to ally with whatever major faction you want. Another great thing is that there are no real good guys and bad guys when it comes to the major factions, they all have their own principles and philosophies which conflict with each other. Plus I think the voice acting is great and the writing compliments it perfectly. Obsidian greatly improved the engine from Fallout 3 by improving things like iron sights, fusing big and small guns, adding survival, adding traits, getting rid of all the useless "Add x points to y skill" and they made the speech skill and general dialogue much better.
Cons? Bugs and glitches. And thanks to Bethesda Harold isn't in this game
Red Dead redemption had a really good atmosphere and you felt like you were in the old west and steadily approaching the end of the cowboys era and moving into modernisation. The plot is nice and long and the side quests offer some really interesting storylines. The character sare all mostly rich and well acted. The co-op missions in online mutiplayer are very fun and there is some nice variety to online. The environments are rich and lifelike and the people in the towns and streets have some nice agendas that seem realistic like watlking into a bar, sitting down and complaining about recent events, to people riding out of town to take a piss behind a rock. Also there is a reference to Red vs Blue in RDR
Cons would be that gun fights can get a bit stale and repetitive and there are some really pointless missions and some sections of the story seemed tacked on and didn't really need to exist. There are also lots of bugs, although I haven't encountered very bad ones. Also the controls I found to be a bit weird and trying to do certain actions can get extremely frustrating like walking through a door and climbing up obstacles. Also for me there wasn't much replay value, once you got 100%(which is pretty easy) there is nothing to do but look at the pretty sunsets.