I LOVE my m17x, however it's an R2. I can tell you though I wouldn't trade this 1200p RGB LED display for a sandy bridge and a washed out 120hz screen.
edit: If the screen isn't that important to you, or you prefer the features of the R3's scree, the crossfire setup they offer is extremely powerful and a great pairing with sandy bridge. I can't speak to the Sager, but the build quality of the Alienware only leaves a couple of things to be desired:
1. Display hinges seem to have about an inch (at the top of the display) of rotation in which they're a little bit squishier than the rest of the movement. I'm not sure if this is intended to make the display easier to open, but it's been like that since day one. I think I'd prefer it to be a bit stiffer all the way through.
2. The power connector is pretty pathetic. It's small, and after not much use will begin to wiggle. I feel like I have to be extremely careful not to damage the port on the computer.
3. I'd LOVE to have a chicklet keyboard, but after getting used to this one it's not so bad. At first, coming from using only Apple chicklet keyboards, I would the skin around my fingers literally going underneath neighboring keys, I never had an incident where it was a problem, but it was a concern at first until I learned how to type properly on this keyboard, now I can do the same speed on this one as an Apple keyboard.
4. The trackpad is pathetic, really. It's not recessed whatsoever, so you are unable to tell when you're actually touching it without actually looking down. It might seem like a nitpick, but after being able to orient myself on the recessed trackpad of a macbook pro, this was an annoyance; albeit a minor one.
As for the plusses:
1. TEN KEY IS AN ABSOLUTE MUST
2. This is the most beautiful display I've ever seen in my life, wouldn't trade it for anything, everything else, including 27" IPS Cinema Displays look completely washed out. Next to my matte 17" Macbook Pro, the red on the Netflix homepage looks grey on the mac and retina burning on the Alienware.
3. As far as retina burning goes, the color profile installed by default seems to be set up by a 12 year old. Immediate calibration is an absolute must, however the tools required for this are readily available and extremely lightweight, it's a one-time deal to fix it, and once fixed... oh good golly, you have to see it to believe it. (this point may or may not apply to the R3 model, I'm not sure about the quality of it's screen)
Overall I have not much bad to say about the Alienware, I think you'd be happy with it, though I couldn't say if you'd be happier with the Sager.
Edited by un1b4ll - 4/2/11 at 12:52pm