why not both?!
cities is 30$ for deluxe n 21 for vanilla on GMG

Cities: Skylines. You'll be able to play it right now, and you KNOW it's good, according to reviews.

FlatOut 2!

Bad Rats GOTY Edition

I'd say buy a game that's already out rather than one that might get delayed again.

Cities: Skylines

Cities Skylines without a doubt.

Cities: Skylines can be played now - you can wait till your next (or a later) paycheck to get GTA V, there is no reason or incentive to buy it on launch.
Cities (especially if you're a fan of the genre) will probably keep you busy for quite a few hours. Which should go a long way to tithe you over untill GTA V actually releases.

GTA all the way! ^^

The main thing to consider is that Cities: Skylines is out, it's good, and it works. GTA V, while it's proven itself a good game on consoles, is yet to release, has been repeatedly delayed, and is developed by the same studio that brought you the world's worst port in the form of GTA IV. I hope it works on PC, but it's really a case of 'wait-and-see.'

So if you're looking for a game this month, I'd have to say, go with Cities. If you're content to wait to make sure GTA V actually works—it's only then you have a dilemma. And by then you may be able to afford both, anyway.