Honestly I didn't experience much difference between my D80 AF and the D300 AF.
This is one thing I can't agree with at all Thomas. Hope you don't mind me quoting you btw. You must have one special D80 as my D80, compared to my D700 feels much slower. There's much less torque for focusing with AF-D lenses as you'd expect comparing a pro grade body to an enthusiast one. AF lock on is also not as snappy even with af-s lenses. Sure it's fast enough. But is at fast as the D700? Not a chance. I know I'm talking FX instead of DX here, but aside from the bigger sensor, the AF systems aren't very different between the D300 and D700.
Personally, I'd keep the D300. I would have thought it would be more accurate and faster to focus in low light, than the D90 is. At least that's what I get with my D700.
To the op; If you can, try and rent a D90 for a concert, do that first. Don't make the mistake of downgrading and regretting it.