To be frank, if you're going with the regular Z, always pick the Incipio mod. You'll appreciate it a ton since the regular Z's battery life is pretty darn lean.
Also consider before your purchase that the cost of the Z plus battery mod is almost the cost of the Z Force, so if you're on Verizon, just consider getting the Z Force. Of course, some people prefer the extra thinness of the Z, or prefer the glass screen, so they prefer the regular Z. Or if you're not on Verizon, then the Force isn't really an option (though it is unlocked and can be made to work on other carriers, if they use supported bands.)
Part of me really wants the battery mod (w/Qi), but it's hard to justify because my Z Force has no trouble getting through a day, so I don't need it for regular use. I do expect I'll need to charge my phone partway through a day when I travel this Christmas, but I already have a 10k mah battery. It's not nearly as nice to use as the battery mod would be, but it's already paid for. If I see a killer deal on the mod, though, I'll almost surely snatch it up.
I have the battery mod, and I find I don't use it much. It does give much better sound, and the phone is still surprisingly comfortable to hold. The kickstand is really nice too. Thing is, though the sound is much better, it's still not great, and most of the time when I want to listen to music, I have a full blown stereo in my car or home that I can connect to and have truly good sound. So most of my use of the speaker mod has been showing off the speaker mod. lol