The Alienware CPU is a very low performance power saving Core 2 Duo SU7300 CULV. 1366x768 LCD screen.
That big 17.3 Acer has a decent Phenom II Quad-Core N950 2.1Ghz CPU and a 1600x900 LCD screen. And a slightly better Mobility HD 5650 video card.
Between just those 2 I think the better choice is the Acer if the big size and higher weight, and shorter battery life, is not a problem for you.
It will blow both of them away as far as gaming goes. I have the G72 and I play everything I've tried on high settings. It overclocks the CPU to 2.88GHZ with included software. If you're not dead set on one of those two you linked then this is a better performing gaming laptop.
that 5650 is a gem in laptops since its a 5730 thats factory underlclocked, which I have and its stable to 684 core and 1096 memory, and thats gives me 34.1 GB/s bandwidth which is approching 460M territory. And that AMD is a tough sell over the I-5 430-560M chips since most games only use dual core chips and the hyperthreading is better than 2 unused cores. so Gaming the I-5 is better, for work like catia/cad + video encoding get the AMD.
http://www.newegg.com/Product/Product.aspx?Item=N82E168... is by a stretch best performance/dollar buy you can get atm. The 360M is only 8% slower than 5870 which overclocked can give you a cheap 460M with a I-5 which has a supported PLL code to OC the CPU too. So in essence with some tweaking you can have a 750$ 3.2ghz dual core /hyperthreaded with a 460M. Buy a mommentus XT 500g and a 8gig ram set and enjoy a 60+ fps on any game at 720P unless its crysis then 45 fps tops.