I'm considering getting either a M17X or a X51. My question is wether I should get a M17X with a GTX 675 or X51 with GTX 555.

M17 with the 675 card. Better GPU with portability.

Well, what do you need from your computer? If you get M17x go with HD7970m, it is a MUCH faster and cheaper card than GTX675m. X51 can be upgraded with HD7000 series and desktop cards are faster than mobile versions, so X51 has the potential of being more powerful than M17x.
Right now M17x with HD7970 is the most powerful option

krishnooof said: Well, what do you need from your computer? If you get M17x go with HD7970m, it is a MUCH faster and cheaper card than GTX675m. X51 can be upgraded with HD7000 series and desktop cards are faster than mobile versions, so X51 has the potential of being more powerful than M17x.
Right now M17x with HD7970 is the most powerful option

I'm totally agree what Krishnoof said :) if you choose m17x 7970m is the best choice :)

M17x = Great portability. 120Hz option is cool but Nvidia only which isn't so great. 7970m is 2x more powerful than previous 6990m. Tried and true platform on its r4 version. 3D optional built in. No need for external monitor, keyboard, and mouse (although i'm not a big fan of the trackpad...).

Note: Great gaming option for people always on-the-go or don't have that much real-estate space but still large enough to be uncomfortable carrying it around.

X51 = Small form factor means it can be moved around/transported. Any HZ depending on monitor/tv connected to. GTX 555 is a little underpower compared to the newer 600 series, but we have yet to see a 6 series that can be used in the X51. Some people have managed to sneak in a 560Ti but a low powered 550Ti seems more likely/stable. 1st version so some may experience issues. Fan tends to run on high alot of the time, some complain that its too loud. Can be fixed with 3rd party applications that control fan speed. 3D if you have TV/Monitor that is 3D ready + Nvidia 3D vision emitter and glasses. Comes with keyboard and mouse, but you would have to lug those plus a monitor if your on the go. Great to bring to a friend's house if you don't have to lug your monitor around as well.

Personally, i'm going to wait till 2013 to see what they do with the X51 R2. I already have the M18x R1, so i'm good in the laptop category. I've maxed out the best upgrades for money and performance on my older Alienware Area-51 X58 so i'm hoping a new Area-51 is in the works! If not, then either an Aurora or the X51, but I don't need a small form factor in my situation!