MS, XY and ORTF (or NOS) and AB all need an fairly exact setup. All have pros and cons.
I'll sum up the most obvious.
Pro: excellent mono compatability, control over width. Great for widening musical instruments, or capturing speech in ENG.
Cons: need special fig 8 mic and a cardioid. Preferably both of same brand and type. Precise setup and often bulky setup (depends on brand/type). Very sensitive to setup errors.
Pro: Easier setup, better centered soundfield, less stereo image. Also available as 1 mic (eg. Rode NT4)
cons: Less stereo image. Not easy to put in blimp.
Pro: Wider soundfield, less center in your soundfield. Easy setup, most forgiving when not precise. Also available as 1 mic (schoeps)
cons: Can leave a 'hole' in the center. Always 2 mics on a traverse. Not easy to setup in a blimp.
Pro: sounds great with cardioids, but much better with omnis! Typical AB setups range from 50 to 1 meter width. Also works brilliant on classical music.
cons: bulky. Works best with high class mics due to distance to source and you need 2 stands to setup, which means you'll have to care more stuff with you. Works best on bigger distances.
When used in close micing: try to keep 3:1 ratio between the mics and the object your recording.
Panning hard left and right with AB/XY/ORTF is fine. Just keep in mind that by NOT panning 'em hard you could get MORE phase problems due to summing. Some frequencys tend to mute or other tend to get ouder. This is why you'd want to be precise on setups.
The NT4 sound is ok, but less versatile than two seperate mics that can achieve the same just by setting up correctly.
Also, why worry about monocompatability that much? If all is well you'd have a phase meter to help you out. Also a mono listen button on your console (or daw) is available. You are obviously going for a stereo setup. As long stereo recordings are not stressed to their limits (eg. overwidening), stereo recordings can hold up pretty good when played mono.
For music: bass freqs tend to translate best when kept mono. Especially on PA systems and radio.
For radio: keep in mind that stereo information (L-R) is separated from the summed signal (L+R); then the previous also applies.
Above would also imply that ORTF would sound less good in mono than XY. But most of the time this does not really poses a problem, due to your recording and mixing qualities. You are able to detect these problems by using the previously stated mono button and phase meter. Oh, and your ears.
Personal: I'd go for two mics. Why? The other day I had a commercial to record. A class full of kids an 1 teacher as a single voice over. The setup was easily converted from stereo(ORTF) to mono. Works fast and the client was happy.
Hope this helps a bit.
Note: The panning issue is not as big a problem as you might think though. Some freqs will phase out, but that also has to do with which setup you are using. Also, the combeffect might hardly be noticable, all depends on the situation and type of setup. Retaining the maximum recorded width you should pan hard L and R. But also, for effect, you could make one sound narrower than the base sound, by not panning hard L and R. Listen carefully to the changes as you go.
Regarding SFX Libraries: Keep in mind/view the phase/correlation meter as you are developing your library. Most FX that are overwidened are 'out-of-phase', will null when played on a mono system. Although I'm not sure what you consider mono playback. A Mac Laptop, although tiny and small, is not mono. Or any other laptop for that matter. Also not all playback system actually add up L+R to create mono. Only one channel is played. Depends really on what you consider to be the playback system. To be sure that your SFX will play everywhere correctly, the best way to go is to check the phase/correlation meter and not hit -1.