Dom, if you put two mics twenty feet apart, you'll end up with a stereo image with a huge hole in the center, even if you use omnis.
This choir recording isn't very easy, I guess you guys haven't read the thread on Gearslutz.
Much depends on the room and the number of persons, I once recorded a choir of 245 persons.
If the room sounds very good, you may have great results with an MS pair or an AB pair of omnis. If the room doesn't sound very good, an ORTF pair of great SDC's can shine. Put them behind the conductor at some 12' high. I have recorded a choir in a church with an ORTF pair at 40' high and it sounded great.
If you have plenty of tracks, use an AB, MS and an ORTF set of mics, perhaps with a number of spot mics.
XY has never pleased me, for I don't like the stereo image of it.
Placement of an AB pair is tricky, you need quite some time to get them right, unless you are happy with mediocre results.
For a Decca tree, you will need three Neumann M50 mics.