The Q6600, because it works with cheaper S775 motherboards & cheaper DDR2 RAM, making it excellent overall value for money.
A Q6700 may be even better(?) value at £127 click here, (with free delivery courtesy of AVF-Scan click here). For the price, click on the 'BUY' link in the list - otherwise it's £3 extra.
If you opt for the i7 920, they have it for under £204 a little higher up. G