A lot of the recorders on your linked page from eBay plug into a wall outlet. I don't think they have those in the pit for Slipknot.
PCM-M10, $229 at B&H. Plus a pair of outboard mics and a battery box.
http://www.bhphotovideo.com/c/search?Ntt=PCM-M10&N=0&InitialSearch=yesThe black one is matte, doesn't get all fingerprinty.
It's just two-track. A live concert is either going to be mono or stereo, so you don't really need more than stereo. Multitrack generally refers to >2 tracks.
You could try just recording with the built-in mics on the PCM-M10. But because you are recording such LOUD shows, built-in mics may overload so you end up with distortion. Also it's kind of awkward to be holding up the unit during the frenzy.
I would suggest getting outboard mics like the Sound Professionals BMC-2 or (even better for loud stuff) Church Audio CA-11 or CA-14--mics you can clip to a shirt or hat, leave the PCM-M10 in a little camera pouch or loose pocket. Run the mics through a 9V battery module (from Church Audio or Sound Professionals) into line-in.
Wow, people record Phish concerts?