If you can afford it, and are a hard core gamer, a dedicated graphic card is better than a integrated card. Secondly consider what kind of games you play. Top tier demanding AAA games, the 15" MBP will play them better. I'd go for the 1GB VRAM configuration. The 13" MBP will play many/most of them at lesser settings, but you might run into a game that does not accept integrated graphics, although I can't give you an example. If you are a casual gamer who mostly plays less demanding games, the 13" MBP should be better than the 13" MB, but if I was making a purchase, I'd have to research that a little. Sounds like that is what you are doing. My wife has a 2008 MB (not MBP) and she plays tons of less demanding games on it including Spore.
Honestly, if you're looking to play games only. Best bet is to go for Acer. I hate to say it, but as I always say, macs just aren't up to the standard for gaming..
Which Acer you talking about and how much does it cost? Granted to get the best performance you have to purchase Windows7 ($60-100) and run it on Bootcamp, but the MBP is a player. Here are the MBP's
tech specs and it sells for $1800 for the AMD Radeon HD 6490M with 256MB GDDR5 VRAM graphics and $2200 for the AMD Radeon HD 6750M with 1GB GDDR5 VRAM. See this
purchase page for a concise features list for each model of MBP.
In addition to getting the Mac OS, you only have to deal with Windows on a limited basis (gaming). I only play games on Windows. As someone who has been using Windows/Mac for the last 20 years, I can assure you, you'll be much happier primarily spending your computer time on the MacOS. Windows, even W7 has lots of bull **** and headaches associated with it that you never have to deal with on the MacOS- honest.
For the OP, if you really prefer Windows, go and do a side by side tech comparison. It is possible to find better tech specs on a Windows laptop at a lower price, but it won't be that much lower and you have to figure in the value of the MacOS which is a 1000 times more than it's $29 purchase price.

When it comes to "gaming quality" laptops, MBPs are very competitive.