I always prefered my cron IV to my cron asph.
Some raves in reviews about the 35 summarit made me curious to give one a try, lets face it these summarits do not look sexy on paper, ...... but BIG SURPRISE: i sold both the crons and kept the summarit.
The 35 summarit works very well with the M8 .. it is not too contrasty but has enough snap.
The 35mm Summarit is especially wonderful for B&W: gorgeous tonal range, creamy, wonderful bokeh.
It draws closer to the 35 cron IV than to the 35 cron asph ........ in some point the files from the summarit come close to the looks of the 28 summicron asph ... modern but not clinical like the 35 cron asph.
The Zeiss lenses are a nono for me because i find them too contrasty with digital, can't comment on them for film: judgement based on 18 & 21mm Zeiss lenses i used with R-D1 & M8.
If you like the look of the 35 cron I (which is expensive: collecter!!) and can live with a slower lens... try a 35 summaron/3,5 LTM ... files look almost identical.
If you like the way a cron IV draws and can give up a little bit of speed ... get yourself a new, coded summarit instead ....