carpa, you seem to be actually interested in all this recording stuff, so let's talk. First of all, I own some cheap stereo microphones and some very expensive ones, but nothing in the in-between range that you're exploring. So I can't say much about the models you mentioned, except that those are all reputable brands. If you're near the point of buying something, let me know what it is and we can look at the spec sheet, just to make sure that there isn't some technical aspect that would make it really inappropriate for you.
About whether stereo mikes are a compromise: Probably, but it depends. They can definitely be convenient, depending to a large extent on what accessories are available for mounting them on a mike stand or suspending them from cables in a hall. Sonically, the main consideration is that most are designed for making "coincident" (single-point) stereo recordings with directional capsules. You might or might not prefer that approach to stereo recording in general. It was a favorite in Europe for a long time, starting in the early days of stereo in the 1950s and 60s, because coincident stereo recordings are inherently "mono-compatible." That is, if you sum the left and right channels you'll get a plausible mono recording, very similar to what you'd have gotten by placing a single microphone where the stereo microphone actually was.
By contrast, if you use a "spaced microphone" setup, because the sound reaches the two microphones at differing times, you can't sum the channels into mono without causing a lot of cancellation and coloration--a general loss of overall goodness. The thing is, mono listening in the IPod era (and the more long-established "stereo-system-in-your-car" era) isn't as prevalent as it was back then. It's still a consideration--but not necessarily big enough to force your choice of an approach, the way it was for some record companies and broadcasters in the 50s and 60s. Anyway the arrival-time differences are less drastic with a solo instrument than they are with, say, a full orchestra and chorus.
It turns out that for piano recording, a "spaced microphone" approach can give you some very tasty goodies, in part because you then get to use omnidirectional microphones which generally deliver much fuller low-frequency pickup than the typical directional capsules of a one-point stereo mike. Also, being spaced apart, the microphones produce much greater, and more sonically interesting, left-versus-right differences ("decorrelated" sound energy) at low frequencies--the main key to spaciousness in a recording.
On the other hand, stereo recordings made with spaced microphones tend to be rather "swimmy" and indistinct as far as the apparent position of the sound source is concerned. But people respond to them in a fundamentally different way from coincident recordings. Perhaps the difficulty in localizing anything makes folks listen more for color, I dunno.
Normally the first choice any engineer would make in choosing a stereo miking technique is whether you want to go for a stable stereo image and mono compatibility or for the more "oceanic" or "enveloping" type of stereo. For piano recording either option is viable. There are also some options in between which I'm quite fond of (e.g. "wide cardioid" capsules spaced a relatively short distance apart). That said, your need to focus on delivering the performance may well force you to choose quick, easy setup over anything else. But depending on how interested you are in the quality of recorded sound, you might want to become aware of these issues.
Finally, let me mention one further two-microphone stereo recording technique that might be of interest to you: M/S. M/S is a form of coincident recording, so it can be made with many types of coincident stereo microphone--though not all, since it requires one of the microphones to have a "figure-8" (bidirectional) pickup pattern. The point is, though, that you can set up an M/S microphone pair at a plausible point and make your recording--then once it's in the can, you can adjust certain aspects of the pickup "retroactively." That's a definite plus IF you have the time to spend choosing your favorite ratio of M to S in playback.
(For completeness: Any other coincident stereo recording can also be post-processed in this way, but the processing then requires one extra step: converting the L/R stereo signals into M/S format. M/S miking directly delivers the ingredients needed for this type of post-processing.)
So that's some background info, which may not help you at all with your practical situation of the moment, but I hope that long-term, it will help equip you to deal with the relationship between the miking method and what you hear and feel as you listen to a stereo recording.
--best regards
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