I can understand the preference for the 9MM. It's lighter, carries more rounds, tad cheaper to fire. From what I am told the stopping power is about the same inside 25-30 feet (speed a little higher with the 9, I believe {force = Mass X Velocity]) but outside of that the 40 packs a bit more. Most encounters at the civilian level are going to be in initial range.
Call me a jingoist, but I am purchasing soon and I am prefer to buy from the USA. You cannot knock the Glock, but I want to give the home team support and they have some AWEsome options out there right now.
I like the S & W/ M & P 9 MM line you can get 17 rounds in the standard mag; no extension necessary and it's a 24 oz gun ! I haven't fired one yet, but from those with experience they said it was solid and smooth... good consistent placement. Also they come with 3 interchangeable grips to give you some options for comfort and stability. The compact version, for c&c gives you 12 rounds in a 21 oz package.
The other option for me is the XD(M). They just started shipping the 9's. I have fired the 40 and it was just about perfect for me. The 9's get NINETEEN ROUNDS (+1) in the standard clip. It is a 32 oz gun and pretty big for c & c, if that's what you are looking for. 8 Oz's difference between the two doesn't sound like much, but it is a 33% weight gain... guess it depends on how long you plan on carrying. Also has interchangeable back strap (rear of the grip) options.
The XD(M) has some great safety features as well. Right now they are coming with two extra mags and mag holster!
I am hoping to pop some with the XD and M & P 9's this week and make my decision. The SIG is a good option is well, but I did not like the feel. If you are really leaning toward the Glock, go try one of the XD(M)'s... it may surprise you! Use the extra $200 to buy some accessories!
I am probably giving wayyy too much info and rambling (it's late). Let us know what you decide and how you like it!