Since there has been some prompting, I'll go ahead and put down my thoughts on sidearms as pertaining to HvZ.
If you have to use a sidearm, you're most likely dead already. Really, that's about it. Sure, they can be useful, and there might even be a time when you reach for one instead of your primary, but if that's the case, it's a secondary, isn't it?

Sure, there are times when it's very nice to have a small weapon that you can access easily, but that was pretty much described under the secondary discussion. I'm genuinely perplexed as to why you'd want a primary, secondary, and sidearm outside of just tacticoolness, and if that's the case, you're doing it wrong. But hey, whatever works for you. I'll go ahead and give more detailed thoughts, but keep in mind, I consider most of these to be for the lulz rather than a proper blaster.
When your bacon is pretty much crispy and blackened, a sidearm can help you take a few Zeds down for a handful of minutes before you join the shambling, supposedly mindless horde. If you are truly using one as a tertiary weapon, then the Powers that Probably Aren't help you should you find yourself in a situation where you need one. I'm not saying that pistol-sized blasters are useless, but rather that they are very rarely preferable to a full-sized blaster (and often are more prone to mechanical failure as well). Still, they can be valuable tools at your disposal, provided you use them sparingly and with great thought.
As always, we're going to be looking at a series of criteria, with weight being given to some moreso over others. So let's go into them!
Size/Ease of Draw: Hey, this is starting off a lot like my secondary criterion. That may be because, to me, a sidearm is a secondary. Go figure.
Rate of Fire: This cannot possibly be overstated. Also mentioned alongside this is handed-ness, or something. The number of hands it takes to operate the blaster. A sidearm that requires two hands is inferior to one that can be fired with a simple pull of the trigger.
Ammunition Capacity: Somewhat less of a concern, but still, the more darts the better.
Range: Even less of a concern, but it's still there.
Okay, this is looking very similar, isn't it? Interestingly, the tiers are going to look rather similar as well. Note that I am including "basic" mods here, since there's really no point in discussing the majority of the stock sidearms.
Best Tier consists of one and only one blaster. It's just that good:
Swarmpistol
What more can I say about this thing? It's a beast, plain and simple. Of course, if you're carrying it in addition to a primary and secondary, I'm going to assume you're carrying around (a) Fenrir and super-Rayven and be more than a little jealous of you.
Highly Recommended... Well, you can probably tell by my earlier position that I really wouldn't recommend
anything as a stated sidearm, but if you must, these are the ones I'm going to suggest:
Snapfire - Unreleased as of this writeup, but previewed glowingly
Nite Finder
Jolt
Retaliator Pistol - Unreleased as of this writeup, but previewed glowingly
Spectre
The Snapfire just seems amazing--multi-fire, semi-auto, what more could you want? A swarmpistol. With the Nite Finder and Jolt, I am going against my own grain a little, but they both boast incredible range and in the case of the Jolt, a very compact size, meaning you can carry multiple primed ones and use them as needed. The Retaliator is somewhat more frustrating, as it definitely requires two hands to wield, but being CS means you have instant access to 18 rounds of foamy anti-Zombie goodness. Plus, I am a complete sucker for Recons. In the case of the Spectre, it's basically everything the Maverick should have been--reliable, fast-firing, and simple to mod. It's just a shame that stock sucks so bad.
I'm not going to really bother suggesting mods here. Take out Air Resistors, replace the springs, get the OMW kit if you can.
Recommended blasters are okay, I guess, but you're already taking something that (in my mind) is a subpar choice, and you're compounding it by taking a less-than-great one as well. Still, you could do a lot worse than these:
Recon Pistol
Proton
Vigilon
The Recon is just a less-good Retaliator soon, so gets bumped down a peg. In the case of the Vortex blasters, they can be useful, but ammunition is again unreliable and expensive. I guess you could combine a Proton with a Shadow Fury and pretend to be some sort of Aspect Warrior, though.
Blasters which are
Not Recommended include anything not on this list, but I'm going to stop and make special mention of one particular one:
Maverick.
There. I said it. Mavs suck. We all know it, and there's nothing we can do about it. They look really neat and I wish they were more reliable, but their inability to fire without jamming means you're better off sliding a sock over your forearm and screaming COBRA! At least then you'll be able to freak the Zeds out a little bit. Best mods: buy something else.

So there you have it, my thoughts on sidearms. Tear 'em up!
[Edit: Added the Spectre.]