Back-ups for us have to be .380 and up (although they may have changed that to include .32s) and have to hold 5 or more rounds. This is more liberal than years past, when back-ups basically had to be a full functioning duty weapon to pass muster for carry. One of our pisotl team guys managed to qualify with a 5-shot Chief's Special, but it was just barely and kind of a fluke, and he was the only one not carrying a mini-Glock, Mustang, P230 or PPK as a back-up.
I used to carry a Glock 21 as primary, so a Glock 30 in a vest holster + zip front uniform shirt was my back-up. I figured that being able to use the big magazines should my primary weapon go down was a big plus. It did tend to print through my shirt, though.
I got forced to switch to a SIG P229 as primary when we went to issued sidearms a few years back (instead of providing your own), so I actually had a back-up gun in a larger caliber than my primary. I usually shoot a slightly higher score on qualification with the Glock, as well, though I have no complaints about my trusty SIG anymore. I was banned by the spousal unit from purchasing another pistol. For some reason, I can get her to bend on rifles (one or two a year), but she hasn't let me buy a handgun in 5 years. She has hers, and thinks one ought to be enough. I only have three pistols.
I have carried the Glock a few times for hairier stuff as an in-the-waistband back-up, when in normal clothes, but have since moved away from that and just started keeping an M4 on the passenger seat when I am going to "expected" serious trouble. If the SIG won't work, a couple magazines of .223 at cyclic rate will probably make them reconsider.
If I can get the wife to approve it, I will probably get a Kahr or Kel-Tec .40 and carry that as a back-up when I go back into uniform in the near future. I like the slimmer lines and think it will conceal under my shirt much more easily. Ankle holsters don't seem to work too well (especially with the way our uniforms are tailored) and inside the waistband won't work in uniform. I have messed around with a few of them, and it seems like either one would make a more concealable weapon. I seem to recall that one or the other had (what I thought to be) a crappy trigger, though.