First off, I'd recommend stereo for a piano no matter what; any style or recording situation. Second, I'd recommend condensers (small or large diaphragm). And third, you never really said how much you're willing to spend. So, I'll give you a lot of ideas. First, in the small diaphragm area, there's
the Earthworks QTC1 omnis for around $2kUS for a matched-pair,
the Neumann KM184 cardioids for $1.6kUS for a matched-pair, the Crown CM-700 cardioids for around the $0.5kUS for a matched-pair, the Elation 201s with inter-changable caps for $0.849kUS for a matched-pair with 1 cap or $1.199kUS for a matched-pair with 2 caps (your choices are Omni, cardioid, and hypercardioid), and the Oktava MC012s with inter-changable caps for $0.619kUS for a matched-pair with 3 caps (omni, cardioid, and hypercardioid). I recommend buying Elations and without a doubt, any Oktavas from The Sound Room.
www.oktava.com
If you don't buy a matched-pair, at least try to buy a pair you are going to be used in stereo miking at the same dealer and same time. The above applies to the mics such as
the Audio Technica AT4041 cardioids for around $0.2xxkUS each,
the AKG C1000s cardioids for $0.2kUS each, and
the Shure SM-81 cardioids for around $0.3kUS each.
Next, there's the large diaphragm area. You could go with
the Neumann TLM-103 cardioids for around $2kUS for a matched-pair, the AKG C414 B/TL-II multipatterens for around $2kUS for a matched-pair, and the Oktava MC319 multipatterens for $0.629kUS for a matched-pair (only from The Sound Room).
As far as these next ones, they do not come in matched-pairs, but they should be OK if you try to get as close as possible, consecutive serial numbers. There's
the AKG C414 B/ULS multipatterns for around $0.8XXkUS each, the Audio Technica AT4050CM5 multipatterens for $0.6kUS each,
the Audio Technica AT4033 for around $0.350kUS each,
the Rode Nt2 dualpatterns (cardiod and omni) for around $0.500kUS each, and the Rode Nt1 cardioids for around $0.350kUs each.
Lastly, also check-out the Beyer mics. There web-site is
www.beyerdynamic.com I would have listed them, but I'm not too familiar with their lines. I own 4 Beyer MCE 93s which are mid-la that I use for toms and are great-sounding instrument mics (in my favorites list for miking acoustical intruments).
Out of all those, I'd probably go for the Earthworks QTC1s out of them all, the Oktava MC012s or Elation 201s in the small diaphragm area, and the AKG B/ULS' in the large diaphragm area; even over the matched-pair of AKG B/TL-IIs (I find them too shrill for most things). Although, the matched-pair of Neumann TLM-103s are pretty nice too; they are my large diaphragm condenser workhorses in my studio.
I hope I didn't just add to your "I don't know which mic to buy" confusion.