So I have the option of getting this game for 360, PS3 and I hope PC. 

My PC was able to run the first Dead Space well enough, how demanding is 2?
My 360 is my preferred platform when getting the game on PC isn't an option.
I'd rather not buy games which aren't exclusives or are the superior version (i.e. FF13) on PS3, but I'm supposed get some sort of limited edition for the same price as the 360 version.
Are there any notable graphical or performance related differences between the PS3 version and the 360 version?

Note: I played the first on PC, but my saves are long gone.
Note: Meant platform in title.

If you plan on doing Hardcore and don't care about Extraction then I would go with 360. 
 
You get an extra save to use when you swap discs. 
 
Otherwise just get the PS3 version, the PC version is apparently missing some content.

For PC, the 2nd is equally demanding as the first. Also, about unlocking the plasma cutter, I thought you just needed any save file, so if you don't have a save file you could easily just start up the game and save it.

Most value PS3
Going for hardcore mode? 360 (or just all the achievements)
PC for graphics and controls (though you might have to configure it a bit)
I ran DS1 just fine, and the same with DS2, though I think my computer was a bit powerful when DS1 came out, so IDK.
 
Sadly there isn't a pc demo out there for you to test :/

One last thing, I forgot to mention that PC currently doesn't have any dlc at the moment, but there are plenty of sweet suits to test out in the game nonetheless.

get it on the ps3

PS3 version is on one disc, and comes with Dead Space Extraction.