All I know of the beemer Z series is the two that appeared earlier in the game. Definitely not my cup of tea, as it seems like the rear end comes around on you at the blink of an eye. Whereas the Lamborghini Gallardos have a wonderfully controlled slight oversteer.
It appears that either car can be used to complete the GT3 series. However with the Lamborghini GT3 needing 20 GC to meet PR requirements, that the GT3 beemer does not, I would like to know if the Z4 GT3 handling is as tail happy as the Z4 M or Z4 Sdrive 35iS? If it's more like the Series 1 or M3, I'll use it to complete, otherwise I'll GC upgrade the Lamborghini right off the bat and use it for the Cup events that have the high PR requirements, as well as all the easier events.
All opinions welcome.