I think that both the Z4 and C63 are tremendous cars in their own rights. The Z4 has the design edge, a wonderfully matched engine to the sublime DCT and a minimalist interior all housed in a cleverly designed, compact shell.
The C63 is well the C63. A precsion sledgehammer if there was ever such thing. Anyone that owns either one Is lucky to have such an automobile. The thing for me is the servicing issue where I live in Australia - too far for the BMW.
And the new C-Class interior looks sooo upmarket. Exterior looks solid too. Only problem its getting bigger and more expensive. The c63 507 over in Oz costs around $200k on the road fully optioned! Z4 35is is around $130k.