I've recorded plenty of cello/piano stuff, and I'm a cellist myself. I've recorded really good players and really good instruments (including a rather famous Strad).
I've used (DPA) omni's in spaced pairs, with Jecklin/Schneider disks, and directional mics in near-coincident and coincident pairs. All I can tell you is that the same thing never works twice in a row. It depends on the room, the players, the instruments, and the repertoire.
That said, I'd probably start with omni's and see if I could make them work. The smaller the room, the more likely I'd be to try them baffled instead of spaced, because standing waves will make the image jump around.
If I couldn't deal with the room, I would switch to directional mics. Owning C460's, I'd probably try the 451's first.
But I'd be prepared to swap them out if they didn't work.
The main thing is to use your ears.
FWIW, here's a snippet of a live solo cello performance I recently recorded. This was recorded in a hall seating about 200 people. The mics were 15-20 feet away. I wanted a pretty focused sound, so I'd planned to use a near-coincident pair of Josephson hypercardioids. But then I heard the pizzicato sound I was getting and I elected to mix in a DPA spaced omni pair to add a bit of excitement from the room.
David L. Rick
Seventh String Recording