if you want to do a few sps than i would go with a 250 and do some atinics on the side. my tank is 24'' tall as well and i have two 250w 10k and one 400w- 20k. i do montis and accros plus i have some softies and lps. if your going with one bulb than thats what i would do that way your less light corals you can put on each end of the tank. a lot has to do with acclimation. i have a sofe pin cussion leather right under my 400w. the only effect is that it's to big and i'd like to move it.
hope this helps a little.
350 gal. 7\'x36\'\'x25\'\'tall, , 3-400w.mh-10k, 2-6\' vho actinic, 175gal.sump, 6ft. 100gal zenia fuge,calcium reactor, kalk reactor and a carbon reactor. 7\' turbofloter 5000 skimmer.