Definitely lens porn ;-)
I managed to bring the two tele-lenses, Zenzanon 4,5/100 and 4,5/135, for the MF Bronica RF645 together. Size comparison. Very similar size at infinity, when focused to nearest distance the 135 is much longer.
If interested, please take a look into my FlickR.
As I know some of you are not interested in hardware photos there are also few pictures made with the 135 in my Bronica album. No direct comparison pics between the two lenses made.
I seem to remember that the 100 focuses down to 1.2m and the 135 to 1.8. That should in theory allow a bit (truly a bit only - should be a difference cca 10%) tighter composition - could you comment how these two lenses compare in real life in the close focusing/portraits photograhy?