The new Z4 looks great, and with a biturbo engine, hardtop convertible along with an improved interior compared to the 370Z, I think I'd opt for one over the new Z.
The curb weight of the Z4 with the turbo engine is 3450lbs, but I do believe BMW does their curb weights a bit different. Regardless though, the increased weight owes itself primarily to the hardtop convertible. I'm also not aware if a regular coupe will be offered.
There is some speculation that the V8 from the M3 might not make it's way into the Z4 due to the economy. But that is all speculation at this point.
Here are the specs
Official Z4 Technical Specs DataSheet and PRESS RELEASE - New 2009 2010 BMW Z4 - ZPOST