Hello thequikad;
The PSP, as you note, has the butt mag release. The earlier versions had the release button relatively exposed, while the later ones are recessed, and therefore not as easy to release accidentally. (For awhile, the PSP was actually catalogued as just the "P7", though now, most of the versions you'll find NIB or used are marked PSP.) H&K no longer has the PSP listed as available in their USA catalogues, nor the P7M13, for that matter, only the P7M8.
The other difference, and the more important one, is that the PSP lacks the heat shield at the upper aspect of the trigger cutout, so that the heat buildup after firing only 10 rounds is much more noticeable than with the P7M8, which doesn't become uncomfortably hot until about 50 rounds are shot fairly quickly.
The noted differences make the P7M8 slightly less "sleek" than the PSP, but the heat shield and the quicker mag release of the P7M8 make it the better choice, IMNSHO.
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