try both?

Well, you'll encounter problems either way... too much roominess in the large room and a colored, boxy sound in the closet. For me, I would tend to go with the big room. You can't do much about the problems in the closet, but if you can use the mic very close and possibly hang some heavy blankets from the ceiling making a 'booth' around the singer, you should be able to get a nice sound.

I'm kinda with David on this... but let me throw this out there as well.

Why... Oh Why... do you think you HAVE to record in the center of the room?!?

The conductor position is where he (hopefully) gets an accurate blend of all the instruments as well as being able to see all of the muso's.

I'd have a tendancy to place a SECOND "room" mic at that position, and possibly try 3-4 different primary placements. I'd start back row-ish by the percussion/low frequency instruments. Then maybe move forward a row at a time to see what you get.

Track maybe 30 seconds at a time and a quick playback... move the mic, lather, rinse, repeat, until you get the most pleasing sound. Then, hone in and fine tune on the final spot. (But I'd also try the conductor's position, too!)

However you do it, I would avoid the middle of the room points at all costs. (front to back or side to side) You are dealing with physics. At those middle points you are dealing with physical nodes, and you'll end up in a node that's either gonna' be a node or anti-node of the frequencies produced. Also, avoid the harmonic nodes... 1/4, 1/8, etc. The quick way to do this is to take a tape measurer and a roll of gaff tape in with you and mark the floor.

Move off the node axis by a couple of feet and you should get far better results.

I'd try also singing to the walls, as well as to the conductor's position. Big rooms can be fun as well as challenging. So have as much fun as you can.

If you find a really great spot, be sure to measure the distances from the walls and WRITE THEM DOWN!! If you get another shot at using the room, you can easily get to work, because you'll already know exactly where to set up.

Oh yeah... just remember to pull up the gaff tape when you're done.

If the instrument storage room is adjacent to the band room,(Usually these are rooms along the side of the band room right?)
Then I would try recording in the storage room, leave the door open, place the singer facing the door, and place an additional mic or 2 out in the band room for a nice verb.

Of course it wasn't mentioned what kind of recording this is going to be. Classical? Opera? Rock? RAP? Solo voice? mixed in with a band? acoustic band? Heavy metal?
Type of song: A belter? a ballad?
That might make a difference on our suggestions

The storage closets are around the room and the room has 3 levels for section seating. The music is acoustic/electronic/psychedelic: http://www.myspace.com/twreckaton

I sing differently in songs, Loud and belting, soft and melodic and then comfortable with a small range of notes.

I would like to apply Line 6 (purple pedal) effects to the vocals for some songs.