The Boxster S starts at 20K above the base Z roadster, but add similar option packages and the Porsche starts to skyrocket in price. I priced similarly equipped Z roadsters and Boxster Ss and the difference ended up being around 30k. A better comparison is the non-S version of the Boxster.
The aesthetics of the Z roadster are a subject of controversy ... But like all aesthetic choices it's rather subjective. That said, anyone who thinks the Z4 is better looking than the 370ZR need their eyes checked
What clinched the Z for me over the Porsche was the sheer uniqueness of the Z roadster. It's rare in my area (even the coupe is relatively rare compared to the Mustangs and Camaros). I can sometimes see at least 4 Boxsters and a couple of TT coupes a day, but I have yet to see another Z roadster on the road and I live in the middle of a metro area with over 1 million people.