What is the difference between the PSP, M7, M8, M10, M13 etc, which one is the more desirable and why? Thank you for answering what I am sure is a stupid question to most of you.

They are all squeeze cockers. and have the same basic design for the most part
PSP- euro mag release cheapest price but just as quality honestly I like this release better
M7- fantasy gun basically its a 45 cal P7 I think theres like 10 of them in existence
M8- psp with a american mag release and a plastic piece in the handguard to help with over heating.
M10- 40 cal P7 I have one and love it but the slide is bigger so alot of folks dont like the looks. Its quite accurate and reliable though. pricey though like 2k-4k
M13- double stack 9mm P7

which is more desirable depends on you.

PSP original German butt heel magazine release

P7 US version of the PSP with a heel magazine release

P7M8 single stack magazine as adopted by the New Jersey State Police

P7M10  .40 S&W caliber P7

P7M13 a double stack magazine holding 13 rounds originally trying to be accepted as a pistol competing in the selection for the U.S. Service pistol which lost to the M9 Baretta

look here http://www.hkp7.com/

P7M7 a pre-production .45 acp caliber that never went beyond prototype in hopes of a military contract to replace the 1911.

Thanks for the breakdown. I always thought these were very cool guns I just never knew there were so many versions and what the difference was between them all.

So the P7M13 is a 13 round   version of the P7M8, I would assume it did that by using some kind of double stack magazine. If so did they have to make the grip of the gun larger to accommodate the fatter magazine? And if that is the case, how does the P7M13 handle and shoot compared to the others? It seems a large part of this guns attraction is its ergonomics. Does the double stack mess with the ergonomics of the gun.

Off handedly it would seem to me, other than the ultra rare models the P7M13 would be the favorite choice unless there is some issue with the gun with regard to my question in the prior paragraph.

Is this so? How much would I expect to pay for a clean well taken care of P7M13? Is it hard to find magazines for it?

Do you want to shoot it or collect it? I f I was getting a ccw I'd go for the P7 single stack. Collectors would be the M10 probably. I own a 10 and a psp and love both and carry both but it depends on what you want. The M10 has the the same grip and mag size and I like shooting it but the slide is different and I dont own a M13. The M13 is a solid choice but try both if you can and see what you like.

never mind

Yes, the grip is a bit larger to accomodate the double stack mag. It doesn't affect the ergonomics terribly, IMO.

Mags for the P7M13 aren't too common but can be found. Sadly, they're around $100 each. Yes, each.  

Average condition P7M13s can be found on Gun Broker for $1300-1400. I've seen unfired ones listed for $1900-2100. Oddly enough, I found an unfired one in a local gun shop & the owner let me have it for a "mere" $1250.    

Yes, it's still unfired.