My first back-up/off duty was a Smith 640. It was .38spl (that's before they went .38/.357 with them) shrouded hammer, satin finish. I loved that gun and shot it well.
Then Glock came out with the subcompacts. I ended up trading in my 640 on a G26. It made absolute, indisputable tactical sense. The G26 will accept my duty mags, so for back-up it's 11 ready to go plus whatever I'm still carrying, versus 5 to go plus the speedloader I used to carry (winter-only when I had the pockets for it.)
Off duty it's still 11 vs. 5 since I don't carry spares off the clock. -And it's 9mm+P from a 3" barrel vs. .38spl+P from a 1.75" barrel.
But... I really miss the 640 and wish I still had it. It carried deeper IWB than the Glock and there's just something I like about wheelguns. I priced a replacement 640, but they're just ridiculous, even used. In that nostalgiac mood I ended up seeing a Colt Detective Special for an absolute steal so I grabbed it. But honestly it's not the same thing as my old 640, 6 vs. 5, exposed hammer vs. shrouded, slightly longer barrel. The Colt's a classic and that's why I got it, but it really doesn't "replace" the little workhorse Smith. (On the other side, the blued finish on the '73 Colt looks better than the satin stainless did on my '94 Smith when I traded it.)
I almost went for an SP101 a gun that I love, but it's even a little bigger than the Glock, and again only 5 rounds of .38 so...
In the end, I shoot the Glock better, it brings more firepower to the party and is much more versatile in terms of on/off duty. When I'm actually shooting it, I LOVE it and have no regrets. When I think worst case, Moscow theatre terrorists, multiple bad-guy robberies, etc. there's no question it's the Glock I want. But when I'm just in a mood to think philosophically about guns, I realize that although it simply can't compete, I still miss my 640.