I'm a D80 user and had the same decision to make like you b4. I bought the SB800 and never regret it. SB800 comes with the built-in bounced card and diffuser as an add-on. I find using the diffuser indispensable. If you want better recycle rate, which i find very useful, just add an extra battery that comes with SB800. I doubt SB600 comes with these features.
Just think that photography is about light and you should have a good speedlight.