Two almost completely different cars. The only things they have in common are the convertible and RWD combo, and just barely. The Z4 M is petite compared to the XLR-V.
I would compare Z4 M with SRT-6 Crossfire, even if it's a reskinned SLK32 AMG because the SLK55 AMG has a V8.
The Z4 M is a very good launcher, but its top speed (ungoverned, even) is nowhere near as high as that of an XLR-V.