The SE 36-72 is much slimmer and uses 52mm filters. It is a push-pull instead of two-ring handling of zooming and focusing for the 35-70.
They both share a longish 'near' limit (1 vs 1.2m for 35-70 and 36-72, respectively).
In terms of image quality both are good, with the 35-70 a tad better albeit it does flare much easier than the SE lens.
You could have both and select the appropriate one for a given assignment. It's not either-or.