I've been a Contax G2 owner since the late 90's with a set of lenses (21/28/45/90). They are ALL amazing, the 45 Planar being the one lens I'd keep if I could only use one lens for the rest of my life. I also have a Leica MP and M2. But I do not own any newer Leica lenses than a 50/2 DR Summicron (but I do have the Zeiss 35/2 Biogon). I like using Leica for more classic, moderate contrast B&W work and my Contax G system for color work and higher contrast, more 3-D look B&W (those with Contax G glass know what I mean). I love it all frankly.
That said I'll recommend you go the G 21 because I'd tell you to one day try the other G lenses and see if you too love them as much as I do. However, if you already own more modern Leica lenses, are happy with them, and feel you'd really not have any interest in the other G lenses (and perhaps even worry about the AF of the G system, which is unfounded but some people complain) then I'd say stick to Leica and just get the ZM 21.