The 135is and Z4 35is have the same engine and are about the same so it depends on the driver:
Wt goes to 135is @ 3439 (about 100lbs lighter).
Hp goes to Z4 35is @ 335hp/5900 rpms (15 hp more).
torque goes to Z4 35is @ 332/1500 (15 more).
0-60 same 4.8 seconds (if DCT for 135is).
1/4 mi guessing about 12.8 for both.
I have had the opportunity to drive both. The z4 I drove was a modified Eissenman version, very fast, and lots of fun. My 135i was modified with 400 rwhp (FBO + Cobb Stage II), very fast, loads of fun, put a smile on my face each time i drove, and very tossible. Both are excellent cars, but since I have owned a 135 I will probably buy a Z4 35is at some point in the future.
I dont think the Z8 counts...the one I saw was powered by the M V-8 engine.