SB600 or 900?
I've been using the SB600 with my D90 for a year now. For bounce flash and very large rooms in a house, it's got plenty of power, and that's with the built-in diffuser in place. It even reached over 20 rows in a concert. How much more would you need?
It may be worth it for you to do the math. Take a guess as to the max distance you'll be shooting and use the flash guide number to determine what aperture you'll need and have available.