I'm really undecided on this, I've never been a person to sell a game after I buy it unless I really never play it and really need the cash, but with the new slightly higher price of games these days, and my ability to logically assume that in light of a sequel to a game I'll probably not play the first one much anymore unless it's got some special replay-value content the sequel doesn't, I'm thinking it might be smart to trade in Rainbow Six: Vegas for some store-credit towards the second one.
The second should have better online and overall better everything, and I've already beaten the first game so the only thing that would be holding be back is the nostalgia ('twas a big game on my list of games-to-get for PS3, and one of the first I bought), and current terrorist hunt levels.
Otherwise there's really no reason to keep it, but those are still two big reasons for me (especially the first reason, I have trouble letting go of things with personal value)

are there any other reasons you guys could think of to keep R6V or do you think I should sell it for money towards R6V2?

It depends how much store credit they give you, if it is really shitty then i say dont sell it.

usually when sequels come out, game stores will give you close to nothing for trade ins of the previous title. On the other hand, I have rsv myself and wasn't too crazy about it to begin with so if anything I would sell the first one on ebay and not even bother with the second.