I like Kes.
I personally don't like 2-for-1s. You'll have to drop someone on your team. Think about it this way: as if you're exchanging this other player out along with Kes for the deal. and then see if it's worth it.
These two-for-one trades are tough to evaluate. What does the rest of your roster look like? Will Briere/Hartnell be better than Kesler/the worst player on your roster? But to answer your questions, Hartnell is returning soon, and Briere might not be as good as he used to but he's solid still.
Another thing to consider, Philly has been slumping and in a strong division (NJ, Pittsburgh, Rangers), and Vancouver has been doing well and in a weaker division(Edm, Calg, Minny, Colorado). Kesler's looking pretty good so far too, so unless the extra player in Hartnell/Briere helps you out a lot, I'd say you're safer to stick with Kesler, the Hartnell/Briere side is a bit more of a gamble. That being said, if you're in a position where you need to take a gamble, I'd take the trade.
Oh yeah, Hartnell should also get you PIMs if you like those