Instead of having CS:GO as a 5v5, have CS:S for it instead. CS:S is much, much, much better for competitive play and a much better game overall than CS:GO.
Not to mention CS:GO already has Gun Game mode built into it.
CSGO all the way for me thanks. CSS felt somehow cheap and dodgy to me. Yes I am simply talking about aesthetics here, pre-baked lighting looked really crap. Game-play wise, I still prefer CSGO anyway.