The Walther P99c natively takes 10 round magazines that come with a finger extension or plain. You can use larger P99AS 15-16-18 round magazines in the P99c with a sleeve that makes it not over insert (thereby potentially damaging the weapons function) and essentially turns the grip into one the same length as the P99AS.
I own both weapons and the inter-operability of the larger magazines is a big plus for me. I have the P99c on my hip as I type this with a 10 round magazine inserted because yesterday I was more concerned with not printing at an event than with added capacity. When less concerned about discovery I routinely carry it with a 16 round magazine or I carry the P99AS.
My favorite aspects of this combination is that the manual or arms (which I think is outstanding) is identical between the two weapons.
In any case, there are some that do not like the P99c and find the grip sleeve annoying. To each their own. I got my P99c 2 years ago when they were going for $339 shipped and after having carried my P99AS for 18 months. I could not resist the deal on the c knowing that it would handle like my full-size P99AS.
I've had zero problems from either weapon. Good luck with your hunt and decision. I understand staying with 9mm. The only 40SW weapons I own have been converted to 357Sig because of that rounds superior ballistics.
P.S. The P99c is available now for $428 at:
Years ago, I had several P99s, including a P99c. I even had the slide on my P99c and 1 of my P99s hard chromed.
At the time, I tried the P99c with the grip sleeve extension - but the edges of it was uncomfortable for my hands when I shot it. It was not the same as shooting with the fullsize P99 grip... At least not to me.
I got outta the P99 as my favorite gun around 2007, and I sold them to buy something else... But, I missed it - and I got another one again back in Jan 2018. I love the gun. And, out of any of the handguns I have, it is the most accurate.
But, while i love the fullsize P99 - the P99c was just so so to me... To me, the rear backstrap was too short. I didn't like how it ended in my palm. That's why I did not get one again, even when they had those super low prices on the 9mm P99c a while back. For that size gun, I do like my HK P2000sk more... The backstrap is a bit longer on that gun.
But, that's just personal preference.
I immensly enjoy shooting my fullsize P99, though