Go with the dual 175W MHs and use either the Iwasaki 15K bulb if on an electronic ballast or the AB 13K bulb if using a magnetic ballast. Put your SPS up top and the LPS down low.
Both the respective setups will provide as much if not more PAR/PPFD than an 250W XM 20k bulb.
AB 175W 13K:
http://www.reeflightinginfo.arvixe.c...&Submit=Search
Iwasaki 175W 15k:
http://www.reeflightinginfo.arvixe.c...&Submit=Search
XM 250W 20k:
http://www.reeflightinginfo.arvixe.c...&Submit=Search