My personal conclusion thanks to my own experience with z88dk and SDCC was that SDCC produced much better code than z88dk. Today I got confirmation from one of the z88dk authors here : "sdcc currently generates code which in general is smaller and faster than the one generated by z88dk [...] it does support a more complete subset of standard C compared to z88dk. It is actively maintained (releases about once a year) and keeps improving.". Don't get me wrong : competition is good and actually benefits to both projects. Still - basically it's a fact that today SDCC is the best C compiler available for Z80 developments.