Buy the one that fits the best. Go somewhere like GPR or DT and try loads on across the budget spectrum, and buy the one that's the comfiest.
I wanted to spend £1000 on something awesome, but the best one for my head was a £300 jobby. You can get the uber cheapy ones that are around £100 and they're fine, but only if they fit your head. Bell and Arai don't for me, but Sparco are very comfy. For the amount of days you're talking of using it, I'd be setting my budget higher rather than lower, but as always get the one that fits.
Did I mention to buy the one that fits the best yet?