The main difference is the SB600 can be set to 85ft whilst the SB900 can be set upto 200ft. I have the SB600 and it a great flash and you can reduce the output if required. The SB900 is the best flash money can buy and can do lots of very clever things and even it's software can be user upgraded if or when Nikon release some. It will also do commander mode which the SB600 can't. If you have one of the lastest Nikon cameras with a built-in flash then that could be used as commander and the SB600 as slave. I would discount the Nissin as they wouldn't be as good.
