I've had a few students at track events that have these cars, and have also had the chance to take a few laps driving these cars at the same events. Ultimately, both are good cars. My ownership experience with cars from both marquees would lead me to have fairly high expectations of their build quality and reliability as well.

The Boxster is one of the best balanced cars CN900 Mini ever made. It feels effortless in how it moves corner to corner. The whole platform is wonderful. Of the base model cars, I actually prefer the engine character of the early 2.5L versions, as I think these seemed to have more in the top than the later 2.7L (though the later probably would be more livable in day to day traffic). The later 987 though is the best of the lot, though possibly too new for the price range.

The downside on the Boxster is that there's not alot of extra room, and that the appointments in the 986 version sometimes feel like they are entry level (which they are, but should be a little better for anything with the Porsche name).

The Z4 is a great car... also wonderfully balanced, but a Vag 15.7 distant second here compared to the Porsche. The Z4 feels like a more powerful car though. I think it would be hard pressed in many cases to keep up with the Porsche because the chasis and braking do seem to be that much better in the Boxster, but the Z4's engine feels very alive and responsive. While the drive by wire aspects of the car work much better than I feared, I think driving the car is still somewhat vague. But this adds to it's somewhat more luxious feel. The BMW gearbox also has a more direct feel.