The SIG M17 is basically a full-size P320 upper (with the addition of an external manual thumb safety) on a carry grip module. The actual grip length of the carry grip module is the same as that of the full-size grip module, but the dust cover portion of the carry grip module frame does not extend as far forward.
The SIG M18 is essentially identical to a carry model P320 except for the color and the external thumb safety. Both the M17 and M18 accept the same 17 round, flush fit magazines that the carry and full-size P320s use, and there is the option of an extended 21 round magazine that works with both.
So when you shoot a SIG M17 you are in essence shooting a full-size P320 in 9 mm Luger. The only differences are the thumb safety, color, and the fact that the dust cover of the grip module does not extend the full length of the slide. None of these should really affect how the pistol shoots. When you shoot a SIG M18 you are shooting a carry model P320 except for the color and the thumb safety. The only difference between the compact P320 and the carry P320 is the length of the grip, which is about 3/8" longer on the carry, and the fact that the capacity of the flush-fit magazine is 2 rounds greater than those for the compact. As I mentioned earlier, a compact can very easily be converted to a carry model by switching the fire control unit and the upper of the compact onto the carry grip module. This is not terribly expensive to do, and takes about a minute once you have done it once or twice.
I own both a full size and a compact P320 although the full size is chambered in .45 ACP and the compact in 9 mm. I also have a carry grip module and have shot the compact with both the compact and carry grip modules and magazines. Obviously, there is some difference in how the full size balances in comparison with the compact. There is a little difference in how the carry balances, with its longer and heavier magazine, in comparison with the compact, but I don't really notice that too much in live fire. The full size has a little longer sight radius which can increase practical accuracy over the compact. I use the medium size grip module on all three versions of the P320.
I shoot the full size a little more accurately in slow fire than I do the compact, but I always seem to shoot .45 ACP a bit more accurately than I do 9 mm in deliberate fire. But I think part of this is due to the longer sight radius. Some people have complained that the full size P320 feels "top heavy". It does not seem that way to me and I like the balance of the full size a lot. The balance of the compact is good but I don't find it quite as natural as that of the full size, but it is the clear choice for carry with the shorter slide.
My main problem in shooting the Glock 19 is that I don't feel as if I can get as secure a grip on the pistol. I use the largest back strap for my moderately larger than average hands, but the blocky grip just doesn't fill my shooting hand as well. Worse, I just can't seem to get a good grip on the slab-sided frame of the Glock with my support hand. No matter how I adjust my grip, there always seems to be a bit of air gap between part of my support hand and the side of the grip frame. This is no doubt simply a matter of my particular anatomy as many people shoot Glocks very well. If I play close attention to my grip, I can get good first shot accuracy with the Glock, but in any string of fire, my accuracy seems to degrade, especially in rapid fire. I find this to be much less an issue with the SIG. I have shot both the SIG P320 and the Glock 19 from rest, and even shooting with support the SIG seems to be a little more accurate than the Glock 19.
