I can't recommend the PPS40, but only because my chubby finger wrapped around the trigger, and pinched me rather intolerably. Otherwise, though, it's not bad. I swapped mine for an M&P40C. (I didn't care about the "thin" issues, and the Shield didn't yet exist.)
The PPS guns use the backstrap and the magazine base as part of their "Hilary Lock", and I don't trust that, either. The backstrap may decide to pop off the gun during a magazine change, but mostly if you damage the magazine base, things get really iffy. This is easy enough to defeat, though, but not "in the field"....
IMHO, the PPS9 would be a great choice v.s. the Shield 9, but minus that goofy lock setup. Add an Apex kit or two, and you get a really nice gun that most people can handle.
(I have an M&P9C, too. Again, not "thin", but it's a fine shooter otherwise.)
No Shield yet - someday - but it appears that I'd like the 9mm at least as much as the M&P9C. I'm not sure if the Shield 40 would be as much fun, but at least it's essentially the same (wider!) trigger as the double-stack M&P's, and I can live with the rest of the problems - namely the thinner grip bashing my hand a bit.
I have no idea why there's a rail on any of these. A bayonet? I can't see a light on the gun for non-LE folks, and would prefer to avoid lasers that can't be more easily (and momentarily) activated.
Just IMHO, of course....
Regards,