Joe, it depends on what you want your tank to be. I get fine coverage and grow all sorts of high light stuff with 2x175W Vertex electronics running Iwasaki 15Ks under Lumenmax pendants. PPFD per watt, this is the most efficient set up according to Sanjay's figures using less electricity than 150W HQI system but putting out more PAR than some 250W systems:
http://www.canreef.com/vbulletin/showthread.php?t=59449
Understand that I have a mixed reef with lower light creatures too and my tank is on the shallow side for a 6-footer. Looks wise no one has ever noticed any dim spots. If you want a full out SPS garden, however, going with three 250W pendants is the better choice. You start getting diminshing rates of return once you hit 400W territory.
You can have a closer look when you come by for you macro later this week.