I have both of these lenses as well as the ZM C Biogon 35f2,8. The difference between the ZM 35f2 and the Summicron 35f2 IV isn't huge. The ZM is better at f2 (less edge fall off and slightly sharper edges too). The 35f2 Summicron is very small and compact in comparison, but I have issues with the construction quality of it. It is a bit flimsy with the plastick'y aperture ring and occasionally I have had f-stops "jump" on them. As for resolution and sharpness, it is probably more dependant on the film and the user. If you shoot mainly bl/w (as in my case) and handheld - you will not see much difference between them. Of course, one of the advantages of the ZM is that it is considerably cheaper as a new lens than the 35f2 IV is as used!
The 35f2.8 C Biogon is a wortwhile alternative to either one of these lenses, if you can live with the 1 stop speed loss. It is reasonably compact - not petite as it uses the largish diameter barrel common to most of the ZM lenses (like a short Planar 50f2). It is very sharp at f2.8 and subjectively speaking - it looks better at f2.8 than both the 35f2 Biogon and the 35f2 IV Summicron @2.8!. It has slightly higher contrast than the 35f2 Biogon too. Not too high, but you see it on the negs. The "fuzzy stuff", i.e bokeh looks OK to me, but then I dont pay much attention to it. It is very well controlled for flare - you REALLy have to work it to get it to flare enough to be noticable.
As in most cases with todays optics, it is more a matter of wallet, ergonomics and personal preference when choosing a lens. They are all extremly good.