The SB 700 has a little less power than the 600 with a guide number of 28 while the SB 600 has a guide number of 30. I don't know that any one will be able to see much difference in the pictures even if trying to fill up a large room. The difference is very slight. The big difference is that the SB 700 has about all the features of the SB 900. At a very reasonable price of around $330 (B&H Photo) it's probably destined to become Nikons most popular selling consumer flash.