TL;DR: Modmic all the way for gaming. Desktop/pro mics can pickup all kinds of "desk" noise, like keyboards (especially if your got a mechanical), mice clicking, movement noise etc.
I have a modmic and a shock mounted Blue Yeti pro on an arm.
The Yeti is amazing, but it picks up everything, and I'm not a loud talker so I have to have it turned up. It's so great, that when my computer ramps up the fans and pump while I'm gaming it picks up a hum through the desk, even though it's on the other side of the desk, on an arm, and shock mounted. On top of that, while I'm gaming I'm moving in my chair, I'm frantically mashing buttons, clicking, the occasional mouse pickup. Normal gaming stuff. All that makes noise. And you can hear it. Yes my voice sounds amazing, and it does have it's place in my setup, but not for gaming.
The modmic on the other hand, people hear ME, and maybe my keyboard because it's a little loud, but everyone is used to that. Plus if I move around and the mic is always there. And the price difference is like a no brainer.
The only caveat I would make to this, is that if you are streaming or doing lets play videos, get something nicer (Like the Yeti, just make sure your setup is better than mine).