[Steve House]

Mic patterns range from omnidirectional through wide cardioid, cardioid, hypercardioid, and super-cardioid as their directivity increases from completely undirectional 360 degree pickup to relatively narrow uni-directionality.

"Balanced" does not always mean "better" nor are unbalanced mics always inferior. It's true that most of the higher quality mics sold these days are balanced but it's not the balancing that makes them high quality but rather the market that they are aimed at. Virtually all professional sound reinforcement and recording equipment uses balanced connections for the increased noise immunity it offers and so mics made for the professional market will likewise be balanced to mate with it. But there are a lot of balanced mics out there, such as some of the music store mic kits sold for garage bands for example, that would be pretty marginal for serious recording use. There's a big difference between the sound of a $75 music store house brand mic and a $2500 Schoeps or Neumann even though both connect with balanced cable.

As to whether to get the balanced 825 or the unbalanced 822, I would expect you'll find no signifigant quality difference at all. Get whichever mates with your other recording and mixing gear. The reason for Audio Technica's cautions about the 822 and phantom power is that it uses a 3-conductor XLR-M connector for its output. This is the standard conector normally used for MONO balanced connections, usually also providing phantom power, but this particular mic has non-standard wiring so even though it appears to be plug compatible, plugging it into a regular XLR mic input won't work and applying phantom power would damage it.

BTW, I see from the Audio Technica site that the 822 has been replaced by an updated version, the AT8022, that can be used with either balanced or unbalanced inputs depending on the cable you use and will accept phantom when used with the balanced cable.

An omni stereo mic would be a contradiction in terms. In order to have two distinct left and right channels, a stereo mic has to pickup sounds coming from left or right, front or back, differently in each channel. If a sound located, say, 45 degrees left of the center-line was picked up equally by both the left and right channel elements, as would be the case with an omni mic, there would be no difference in the two signals to generate the stereo effect when listening to the recording. Two overlapping cardioid directional patterns, one angled toward the left and the other angled toward the right, would give you the different sounds coming from the left and right speakers that you need.