They're pretty much the same thing. Also pretty much the same as the PVM-195X series as well. Sometimes they switch phosphors and whatnot, but nearly identical.
The one change you will notice is the OSD (On Screen Display - menu) is vastly different on the L2. Same stuff is in there but in a different menu structure. My 14L2 has the not-so-uncommon problem of the menu liking to disappear for no good reason (think I made a post on Shmups about this and found a bunch of other people, and other posts on other forums that had the same thing happen to them). Ideally you shouldn't be going into the menu constantly, but when you need to and it doesn't work it's a PITA.
For condition, there's no hour counter on either of these so there's really no way to tell if one is on it's way out or not. The easiest deciding factor is simply going to be which one you find first and if the price is decent (I've seen WAY more M2's and L2's for sale around me...).