My answer: No, I would not buy a Mac or an Alienware for gaming.
If I were gonna get a new gaming laptop, I would get an ASUS G55 and then reload it with Windows 7. Or wait and see if a newer one comes out.
Back in 2010, when the 330M-based machines came out, I listened to people here who swore up and down that the 330M was sufficient for gaming. It wasn't. It was a dismal failure; I ended up buying an ASUS G73 (the cheapo version at Best Buy) for about 1/3 the price of the top-of-the-line MBP, and it ran rings around the Mac. Because it had an actual non-sucky GPU, mostly, but also because it was large enough to have reasonable heat dissipation without relying on super high fan speeds.
The current gen is pretty livable -- the 650M's not awful or anything -- but there's still a serious heat issue just because the machine is so very small and hasn't got enough mass or surface area for heat sinks. So in practice, I'm using a G53 (GeForce 560M with 2GB of VRAM) for gaming, and it is in actual use faster under heavy graphical load, to say nothing of actually having enough VRAM to run stuff at higher settings without much trouble.