So tell us about the comparison.
There is no short answer to your question, but I'll try and hit the highlights,
I owned a Z4 3.0 w. sport package (18" wheels / sport mode for throttle and steering).
Like all of the new BMW exterior designs you have to get this car in the right color for it to look good and still it will be polarizing.
I love the looks of the Z4 and the way the light plays against the dramatic lines, but only in a couple of the offered colors.
Mine was sterling grey / tan / tan
Side by side I like the looks of the two cars about equally. I like both car's interiors. The Z4 is very modern looking, but the new Boxster has the Audi-like quality to everything you see and or touch. I especially like the way the black interior of my Boxster is highlited by all the chrome and/or chrome look bits. Very upscale. My only complaint would be the center console. Buttons too small, too numerous and plastic looks a bit cheap.
Unless you spec' the sport seats in the Z4, even the base model seats in the Boxster, which are what I have, are far superior for sport driving.
I prefer the steering wheel in the Z4 to the standard wheel in the Boxster, but the sport wheel option in the Boxster evens things out here.
Driving position can be dialed in beautifully in either car, but the closer pedal grouping of the Boxster makes heel-toe shifting a bit easier.
Performance:
The Z4 can be a blast to drive, but can also be a drag depending on what you are using it for.
Throttle response (in the Z4) is amazing in sport mode and the car feels very smooth and quick to red line in all lower gears and makes wonderful noises getting to redline. Clutch engagement is a bit tricky though and requires precise feathering to get the shifts down smoothly. Even once I had the clutch delay valve removed (dreadful little device) it took a while to drive the Z4 without the occasional rough gear shift. The Boxster's throttle response is a bit slower (I don't have PASM or Sport mode in mine), but the shifting and clutch engagement is simply fantastic. It is nearly impossible to get a *bad* shift. In this area the Boxster wins hands down.
Steering feel is also vastly superior in the Boxster. BMW's Electric, speed sensitive steering needs some work. Perhaps it's just a matter of re-writing the code, but whatever it is the Z4 has a slightly vague and artificial feel to it's helm. By comparison the Boxster's steering feel is brilliant. Absolutely perfect!
The Z4 does steer very quickly and certainly goes where it's aimed... Right Now... though so it still is a blast to squirt around corners and curves even though the fun is hampered a bit my the overall feel of the mech.
BMW's DSC (Dynamic Stability Control) is much more invasive. When it kicks in you REALLY feel it. Porsche's PSM is much more subtle and progressive and definitely gives you a longer leash.
Ride quality is so much more mature and dialed in on the Porsche that this may be the area of biggest difference b/w the two cars. With the 18" rims and sport pack, the BMW jiggles and bobbles over rough pavement and makes you feel like a kid in a slammed Honda. Not a problem on smooth pavement, but on areas with pot holes, cracks, expansion joints, frost heave etc...the BMW does lose its composure rather quickly. This bothered me the most when I used the Z4 to commute 35 miles to work over rough I-40 segments. If I left work with a headache I was sure to arrive at home with a migraine!
The Porsche soaks this stuff up like an E39 5 series! Perhaps it was necessary to make the Z4's suspension this "sporty" in order to get such high skid pad numbers from a front engine car in which you practically sit on top of the rear axle....?
I have the 18" rims on my 987 S though and having test driven a non-PASM car with 19's I feel that the 18" rims are The way to go on the 987 unless you ar ealso plumping for PASM. The 19's look wonderful....much better than the 18's, but I felt they spoiled the ride and therefore made the car more nervous....more like the Z4 on 18's. I wish Porsche had given new Boxster owners the look of the 19's in an 18" size.
I could go on and on, but I'm sure most won't even get this far so I'll stop here. I'll just say that overall both cars are fun to drive and, in my opinion, both look great. But again, one of the marks of a truly great body style is when the car looks great in any color. I have yet to see the 986 or 987 look less than stellar in any color. The Z4 looks it's best in 2 or 3 of the offered colors and, in my opinion, actually looks really bad in a couple of colors.
One very important thing to keep in mind though. Due to the Porsche's clearly superior chassis and steering you have to drive it at very high speeds to really get your oops-I almost-lost-it-in-that-curve-back-their adrenaline flowing. With the Z4 you can have this kind of fun at merely 20 miles an hour over the posted limits. With the Porsche you have to be clearly in license losing territory to really get your heart pounding.
This is an important consideration, but further speaks to the superiority of the Boxster design.