For me, I prefer the Gen 4 27. The grip does not bothere me. Some people say the recoil is a little brisk, but i can't tell the diffrence between the 23 or 27 in that regard. The 27 Conceals perfect with the 9 round mag and sometimes i carry a spare Glock 22 mag for more fire power. The 27 is just as accurate as the full size .40's. For me, the 27 is the best balance when it comes to Glock .40's; a true everyday, every situation pistol. The 23 is awesome too, my bro owns the 23 and I almost went with the 23 instead but liked the 27 more.
But as others have said, you will have to hold it and shoot it before you buy it.