I ran an SLP for 2 years and just sold the 2nd of my 2. I'll be shooting a VM next year.
The benefits of the SLP are primarily slug accuracy. This comes from 3 things. Stiff barrel, a one piece mag tube and rifle sights. The rails were cut off on my SLPs and the stocks modified to fit. They do not cylce anything and most are finicky in the first few thousand rounds. Welding the lifter affects the speed a tad and the dogleg spring needs more frequent replacement. SLPs last about 10K rounds or so, less if you seal the pistons, more if you run the heavy piston.
Shooting a VM in a few demos, it was obvious that it is has the least recoil for the same round and with the same fit. But, I did not want to give up the benefits and reliabilty of the one piece tube. Now I have a VM with a one piece tube and the slug accuracy is on a par with the SLP, even with the vent rib sights. The 3.5" loading port is faster, especially with the TWinS and Quad loads. The gas system is simple and clean with no springs or seals to wear out. The firing pin retainer issue has been rectified.
The Sportsman VM is now announced and should be available for a street price of about $750 by the end of the year. If you want to add a one-piece tube, that will be a cheaper option. When you look at the top tier of the 3Gun Pros, the ones with a shotgun sponsor are shooting what they are supposed to, the rest are shooting what they want to. Most of those are shooting VMs.