Does anyone know the difference between the two?

Someone adding a letter to the designation to sell more guns. There is the EVO S1 pistol and the EVO S-1 carbine.

Has anyone actually ever seen one marked PS1? Serious question, I haven't.

Anyway, "PS1" is usually referring to the 2015 model (More accurately: SKU 91350, and date coded "15") which has 14x1mm muzzle threads.
SKU 91351 is the date code "16" model and has 1/2-28 threads.

I do believe they are all marked "S1" though, so check the date code.

(BTW: the first Bren pistols WERE marked PS1, and the new ones with updated threads are marked "S1"
Also, all carbine models are also marked S1.)

I have one of the original Scorpions, and yes, it is marked "S-1".
Yes, the BREN was marketed as the PS1 when it first came out - here's the picture I took of it's debut at SHOT 2015.

There's one at buds marked "PS1" on about the third or fourth page of CZ handguns. It's the only place I've seen one marked ps1. After some research and not finding anything on the ps1 model, I'm guessing it's just a misprint at Buds.