Well the thing with Mercs 2 it's a gamble. People have complained about major glitches in it... I never encountered them, though. I still think Mercs 2 is one of the best open world games for just blowing stuff up and generally f***ing around in terms of old school blow-s***-up action.
Both of them have demos so try them out? --- I am the f***ing warrior... Scandal said so.
I like both, own both but if I had to choose I would say Red Faction: Guerrilla. It was just a better game in my opinion. Mercenaries 2 kinda lost something that the first one had and for all it's supposed open world-ness it really boxed you in most of the time and had a lot of arbitrary game play designs that really reeled in the potential of just plain fun it could have been. I guess my biggest hang up with Mercenaries 2 was that is just never lived up to being as good as Mercenaries one despite all of it's supposed "improvement", as it removed and broke a lot of things that that made the first one fun. Don't get me wrong though, I still enjoyed the game, but I couldn't outwardly recommend it despite my liking it as opposed to other games in which I could easily do so.
As a worthy side note and honorable mention I would throw Just Cause 2 into the arena as well if you don't have it TC. It is another great open world destruction game that makes it feel fun to play and actually gets it right better than both of those games IMO. --- GT: Bool Zero / PSN: BoolZero / Wii:Who uses Wii codes, really?
RFG, it just has a level of polish other "sandbox" games fail to attain. While I can't speak for the current climate, RFG was still somewhat active on the MP front. Enough to find a game of Team Anarchy with out too much waiting. That was right before Armageddon's release. --- RIP Red Faction 2001-2011.