I have and use both the SB-400 and the SB-600. Each serves a purpose and they both perform very well. The SB-400 is excellent as a daytime fill-flash - it recycles faster than the built-in flash and the SB-600, is small and lightweight and is a lot more portable than the bulkier AB-600. It shoots what seems like forever on 2 AA batteries and can be tilted vertically. It's a great flash to bring to the beach and park where you want to combat those harsh shadows you may otherwise get in strong sun.
The SB-600 is another beast. Quite more power and can be fired off-camera as slave from the D90 and up Nikon models, without any extra triggers. For the purpose you describe - blasting the background of something, this model is more appropriate. It comes with a stand too.
Good luck with your choice!