Widebands went the way that everything else is going these days. The absolute best was discontinued and an inferior product replaced it. The best consumer grade wideband was the NTK #91101. It was replaced with the 96604, which is ok I guess, just an average wideband.
Ballenger somehow worked out a way to continue making the original but with their branding, it's come down in price a lot, it used to be crazy expensive compared to the original. See here:
Clicky
Rumor has it that there is now a third version being released #90067, it's supposed to be able to use the same great sensors as the original. But I haven't heard any real world feedback on it yet.
As mentioned above, you never want to leave a sensor in the exhaust without your controller plugged in. The NGK/NTK sensor mentioned above would be a great "portable" solution.
You didn't mention which EFI you want to run...With Mustang EFI you can wire the wideband output into the EGR circuit to datalog AFR (or more preferably, Lambda), so any wideband with an analog out will work. Innovate's products work pretty decently if you want to log through USB. I kind of like the MTX-L, but it's not as awesome as the original NTK. I believe that most of the aftermarket TBI units are using native wideband.
Another thing to mention is the placement of the sensor. Wait until you have made your decision and have your sensor before you go adding the bung. Different manufacturers specify different lengths from the collector for sensor longevity and accuracy.
