In Canada it is a 2000 $ option ... arrrgh !
When I was asking myself the same question I had the chance to sit in some X1 with both interior. I would say the type of seats should also count in your equation. The leather is "sliding" on the standard seats. So, if you can't afford the sport seats, I would recommend you the leatherette. The std seats will keep you in place better with this no cost option.
The sales person had made me realized that the leather was not "ventilated" in the X1 so I would'nt gain anything in the "sweating" side of the thing.
I have my X1 for almost a year now and I have gone through a fantastic and hot summer ... Neither my wife or me had any complain about excessive sweating at all.
But, go and try both. It's not because we like the letherette enough to save 2000 $ that you will too. I'd say save the cash for a sport pkg, a panoramic roof and the light pkg ( directionnal xenon lights ). Those 3 are really upgrading the ride.