I would rather use VocoPro than Behringer's wireless mics. I have no experience with the
Line 6 stuff, but I knew people using their digital stuff for measurement mics. Personally, I would shy away from WiFi bands, though I have no solid evidence to back it up. I just feel like there's too much crap spewing RF in those bands. If you're looking for something with decent frequency density, I think ULX-D is the best bet right now, though exponentially more expensive than the Behringer crap.
Especially in
theatre, wireless mics are the one thing you shouldn't cheap out on. The last thing you want is to have a mic dropout when Diva McLeadingactress is onstage for her penultimate solo. The range on the cheap stuff isn't that great, they don't sound particular good, and they break easily. A minor pet peeve is the cheap ones use 9V instead of AA batteries.
Shure has relatively cheap systems with the SLX series (they lack in features, ie
gain settings, RF
meter, but they're pretty durable), and the
Sennheiser stuff isn't bad either (my preference is the 500 Series and above
Sennheiser, but most venues around here are using
Shure ULX or UHF-R).