In a 9mm, there are very few weapons better than your Glock 17. High capacity, extremely reliable, and a proven design. With proper ammo, performance comes close to .45acp.
If you absolutely, positively must have the newest and perhaps best 9mm out there, check out the brand spanking new Smith & Wesson MP or M&P. This pistol might possibly be the start of a new generation in plastic nines. I won't spend my money on it, or any other 9mm, but if you're on the lane next to me at the range, I might just be really nice to you and ask if I can put a mag-full downrange.
.357 sig was an advertising gimmick. Nothing more. See Fackler or Roberts.
I found the recoil on .40 cals to be very sharp, no matter the platform, but you should be aware that this sensation of sharpness is very subjective and not felt by everyone.
In ballistic gelatin tests, 230 grain .45acp is still the way to go. Which gun? Depends on your budget and if you prefer single action, double action, or double action only. I prefer H&Ks, but many Americans have a very powerful fetish for Saint John Moses Browning's model 1911.
My advice? If your Glock 17 is reliable, keep it, make sure you load it with excellent quality ammo, then forget buying another pistol. Instead, use the money you saved for training. Then save up more money for more training. Then have your wife get a 2nd job, so you can afford more training. Win the lottery, so you can train every day!
Remember, there is a triad when it comes to personal defense. This consists of weapon, mindset, and training. You need all three to win a gunfight. Do NOT be deficient in any.
BTW, you do have long guns, right? A shotgun or rifle is far superior to any pistol.
+1 on the idea of sticking with your Glock 17. The advantage offered by the 357 Sig is too minimal to make it worth your time to change over to a new caliber.
If you are stuck on Sig, and aren't wedded to the idea of a double stack magazine, I'd take a look at the newest Sig 220 variant in 45 auto. That is the one I really want.
Right now, on my nightstand lies a Glock 17 3rd generation variant with a Surefire tactical light installed, and 19 round +2 9mm magazine installed.
I shoot the Ranger 127 grain +P+ in said handgun. It's very very close to the 357 in power, and I have 19 rounds in the magazine.
On the street, I carry the Glock 26 subcompact with the same loads.
As I said, the only Sig that really turns me on is the 220, and I'm waiting on the new Commander sized version to come out that I read about yesterday.
A .357 SIG 125 gr bullet is as effective as a 125 gr. .357 mag. bullet fired from a revolver if that is your cup of tea. If 9mm is what you are used to, you will find the .357 in any format to have pretty sharp recoil. ,45s are big, pushy feeling pistols when they recoil but quite manageable. While I like .45s, .357 get the job done while 9mm is about like a .36 Special and a bit on the wanting side as a fighting round. Properly applied any of them work OL. (practical experience, not gun writer stuff.)
The S&W M&P semi-auto is only available in .40SW at the present time and is still a bit hard to find. The 9mm will be coming out a later date. The jury is apparently still out as to when it or if it will be available in .45 ACP.
I favor the Sig 226 in 9mm and looking for a Sig 220 in .45ACP. I've shot the Glocks and like them; just like the Sigs a bit more.
Yes, a DPMS AR-15 clone. (The RFA2-PCAR16)
I agree with MyDogAllah (love that handle!) about the value of training. A hit with a .22 generally beats a miss with a .44 mag (unless you scare the guy into a heart attack!)