For the money the SLG is a good gun. Easy to maintain. It is a sear tripper so it is loud and has some kick. I've had one for a couple years and it has yet to let me down. I've found that anything over 13bps raises the probability of breaking paint because the bolt is not particularly gentle. Perhaps an aftermarket bolt could fix this, but do you really need to shoot over 13bps?
Next is the PMR series. Nice spool valve gun that is quieter and gentler on paint. Has a lot more o-rings so maintenance is slightly more involved and a leak could be harder to diagnose. But it feels much better shooting than an SLG. I own one of these as well.
And of course the PM/DM series would be the best among those you mentioned. You're looking at a tournament level marker. I owned a PM7 but sold it because I didn't feel it made me a much better player than my PMR. Keep in mind I'm old and I only play woodsball, and I also sold that PM7 at a healthy profit.
If you're in a good position money wise or play a lot of speedball, it may be worth it to you to go with the PM/DM line. It just was not quite worth it for me.
It all depends on your budget. A 2008 PMR costs about twice as much as a 2008 SLG. A used PM8 costs about twice as much as a used 2008 PMR. That's approximate of course, but not way off. So you could get 3 or 4 SLG's for the price of a PM8. The question is, how much are you willing to spend?
Approximate used prices (stock). These are my approximations so if anyone disagrees I don't care.
2008 SLG - $90 to $130
2008 PMR - $180 to $220
PM8 - $300 to $400