GMG sales or wait until steam summer sales ?

half and half?

If it's 75 or 66 percent off, chances are the summer sale won't be better, or not much better at least, so you might as well buy them now. If it's only 50 percent off, take it on a case by case basis. 50 percent off the Van Helsing game, for example, is a great deal, but 50 percent off Hotline Miami will likely be beaten by the Steam Summer sale.

For anything less than half off, definitely wait.

"had to stop my car for 2 days to buy the Total war franchise"

Well... maybe you should, you know... not be spending money on video games then.

If it's for you, probably gmg sales will be cheaper.
Remember to use voucher for extra % off.

GMG usually has better deals, steam has some interesting deals on indie games every once in a while though.

When I go to buy a Steam game from somewhere other than GreenManGaming or GamersGate, I always check to see if it is available on those two, as a Steam key. That part is important because most games aren't Steam at either of those places.

Unless the deal is really great and you will likely not see lower, I'll usually wait on any Steam games I see at those two places. At GreenManGaming you get the 20% off coupons and at GamersGate, you get blue coins, which can be used to purchase other items. You get 5% back on each purchase, 50 blue coins for reviewing an item (100 if you are first), and there is a 5% off coupon, as well. On top of that, they give you experience points which can increase your level on the site. I have a 20% off discount there, as well, so that helps. Gamefly also does good coupons, if you find a deal.

My rule is 75% off a good base price, to start with, on almost everything. I make exceptions, but usually that is the way to go. 75% off becomes not good enough if you can do better with coupons and cashback. Since Steam typically does standard sales and their coupons tend to not be able to be used with a sale, you should check and see if you are getting the best deal possible. Unless you are made of money, every cent saved counts for something, including more games, later on. You actually have to verify you can get the Steam key elsewhere. You can't for every game.

A couple of nice places for price histories are IsThereAnyDeal and [SteamGameSales].(http://www.steamgamesales.com)

I say Steam summer sale.

I just bought Tomb Raider for 13,60€ with voucher @ GmG.
This is cheaper than any keyseller site I know. Don´t think this one could get any lower^^

the steam summer sale is not near enough to have to ask yourself this, its not even june, and it will be in july..

neither, wait for humble bundle sales

Amazon and GMG regularly have much better deals than Steam.

The Square Enix deals (apart from Tomb Raider) on GMG right now are kinda crap. I have all the deals posted here.