Which game should I buy first?

Also, which one will give me more play time? And is Uncharted 2's community still strong?

more play time? Well Uncharted 2 is about 8-10 hours long, RDR is at least double (single player of course, both have multi)

Tough call. While I want to say RDR, Uncharted 2 was last year's Game of the Year, and now is at a reduced price. So maybe if you care about saving money, you can get Uncharted 2 now for cheaper, then by the time you get RDR, maybe it will have had a price drop as well. Because truthfully, let's face it: in the long run, more money = more games.

Seeing as RDR is new, and also an open world game, it should give you more enjoyment. But I have no idea.
 
Useless answers!

I really liked Uncharted 2 and haven't gotten my hands on Red Dead Redemption yet so I can't say much about it. 
Uncharted 2 was had a great story and great characters and I was entertained all the way through it (just like with the first one), and the online wasn't terrible and also kept me entertained for a while. 
Something about the online for Red Dead Redemption just looks like way more fun to me, so I would probably pick that if I didn't have any of the games. 
 
but again, I already own Uncharted 2 but know nothing about how Red Dead Redemption plays or feels or how good the story is  
The quick look was a really quick look at the scenery though, at least it looks pretty, so that's a good start =D 
 
I guess my opinion doesn't really matter, but there you go, fake linky =D

RDR is the longer game, but UC2 is really cheap now.  If your interested in MP, RDR's will probably be more active as its only just come out even though they just released that DLC for UC2.  Of the two, tough call, I'd probably go with UC2 as its cheap and amazing.  My policy is to wait a month or two after release to get a game (unless I'm really after the MP) as UK prices are dropping fast at the moment.
 
I've gotta ask someone, whats up with Little Red Riding Hood avatars?  I've obviously missed something.
 
Edit: there are people talking about RDR's mp being a bit broken at the moment so maybe get UC2 while you wait for them to patch up RDR.

Uncharted 2 was giantbombs game of the year Igns game of the year 2nd place at gamespot  and gamekraft.webs.com game of the year(yes I had to plug my own site lol )

This poll seems even more redundant than the RDR vs Alan Wake one.  I think we're going to see too much downvoting of Uncharted from non PS3 owners.
Also note that RDR hasn't even been released in most countries yet may factor in though..
 
Personally, I'd go with RDR I think it has more replay value and Uncharted is only going to drop in price as it's quite old at this point.