As far as the Mid size Glocks being about the same when it comes to concealability as the sub compacts, that is not true in my case.
I was carrying a 23 everyday for about a year and a half until I bought a 27, the short grip on the 27 makes it print a hell of alot less than the 23, there really is a big difference when you use the stock mags without any extentions.
Anytime you are going to buy a gun that will be used as a CCW you will end up giving a little to get a little. What you loose in grip comfort you make up in concealability. The 27 to me is the best CCW on the market when you combine the size,weight,capacity and just raw power that it posseses. I have said this in a million other threads but I am going to say it again, you would be hard pressed to find a better CCW gun.
As far as getting the .357sig sub compact, I went the other route. The availability of the .357sig ammo was not the best in these parts and it is a relatively new caliber that has not had enough time in my eyes to get a good reputation or atleast assure me that it is going to be here in ten years, or is it going to be a special order item like the 44 special (one of my big mistakes), those were some major questions that I asked myself and to make a long story short the end result was a Glock 27 with a .357sig barrel on the side.
I enjoy both calibers but love the .40 all around. An after market barrell (barsto, the best) will run you about $150 or so, not bad considering the return ay?
Good luck in your decision.
~Jason