The prefacelift 3.0i is quicker than the 2.5Si. It's also not a particularly great facelift. The Z4 is quite uniquely styled - everything fits the lines of the cars, ie the back lights. The facelift seemed to be a facelift for the sake of it - the rear lights on the facelift, CELIS as per the E39/1 Series Coupe etc, just look out of place and don't seem to fit the lines of the car.
You'll get a pick of really nice 3.0i's for the money, too.