FYI, because of the way the grip sits relative to the barrel on the Sub2K, the actual grip of the one that uses the G19 mags will actually be as small for your hand as a G26 pistol. Likewise the KelTec for the G17 mags actually has a grip size of a G19.
I had one for G19 mags. I hated the grip. If I were to do it again, I'd get the G17 model and then cut out the sides and front of the grip so I could still use G19 mags (assuming that's your sidearm you want to coordinate with) and then the backstrap would be the much more comfortable length of the G19 pistol.
Even though I don't carry my Glock 19 or 17L, my matching carbine for them is the take-down Ruger PCC with the Glock magazine adapter. Not nearly as small or fast-folding as a KelTec, but it's a FAR superior carbine, and magazines don't go through the grip, so you can use ANY Glock mag.