As Shane said the difference is the COL they will handle, M118 P-Mag at 2.835 and a standard gen 3 P-Mag at 2.825 for the mags I have. They both work the same for me in the two rifles I have that run P-Mags. The M118 mag is a 25 round mag but don't really use it due to being so long, I actually use a 10 round P-Mag the most because it works the best when shooting from a bench;
https://forum.308ar.com/topic/12319-magpul-m118-gen-3-p-mags-review/#comment-169443
Palmetto State Armory runs a special every so often that bundles P-Mags and some ammo but I don't see it now, still have decent prices though;
http://palmettostatearmory.com/catalogsearch/result/?q=p-mag+lr
I've also got some good deals from Primary Arms;
http://www.primaryarms.com/ar-308-magazines/c/110700/
The Lancer mag gets top reviews from most everyone that tries them, here is a thread with some close up pictures of one. The bolt catch height issue I mention turned out not to be an issue, never had a problem with the Lancer, never had a problem with a P-Mag either though;
https://forum.308ar.com/topic/12327-lancer-l7awm-20rd-magazine-reviews/#comment-169584