The PMR is one up from the SLG in the proto range.
I dont see there being a major, noticable difference between the 2 unless you are going for the Limited Edition version of the PMR, which comes with alloy back cap, 2 piece barrel and a few other little changes which make the marker feel a little more high end when compared to the original version, while the plastic frame of the SLG never lets you forget your holding a budget marker.
The other difference is that you can get an ultra lite frame for the 07 PMR which isnt available to the SLG 08, just for this reason the PMR comes out favorite for me.
I had an SLG as my first marker and then upgraded to a LE PMR with an UL frame and i wish i had kept it as a spare as it really felt great to shoot.