The E8500 would probably be the best overclocker, since it has a higher multiplier than the E8400. It's not guaranteed, though, as every chip is different.
However, the Q9550 would most likely be fast enough for whatever you throw at it (unless you're running two or more high-end graphics cards in SLI / Crossfire), and it should still overclock decently. Furthermore, if you're looking to keep the LGA775 system for 2+ years, you may start to see a number of programs that benefit from more than 2 cores.
If the Q9550 is within $10-20ish of the E8400 or E8500, I'd definitely buy it. It's more future-proof.