Having never tried the Zeiss 85, I can only go by reports I have read. Apparently the Zeiss lens with its moving element is better corrected for close-up photography than the M mount lenses you have mentioned.
I have both 75 Summilux and 90 Summicron Apo ASPH. I like them both for different reasons, although neither have moving elements for close focus correction (the 75 Summicron does have a moving element). The 90 is a lens with great brilliance, and is better corrected for mid and long distances. The 90 pre-ASPH lens is a very fine lens without some of the snap or contrast of its ASPH stable mate. The Summilux, a 25+ year old Mandler design is a different animal: a fast, short "portrait" lens that is sharp wide open, although has less contrast at 1.4. Stopped down one stop, the Summilux gains contrast; to my eyes by f4 the Summilux is as brilliant as the 'cron ASPH 90. The Summilux, even moreso than other mid-teles, does take some practice focusing wide open in the near range.
One thing to note: the 75 Summilux is shorter than the 85/90 focal length. I really like the 75 focal length, while others feel it to be a stepchild, betwixt and between 50 - 90 lengths. Some do not like the 50/75 combination frame lines, although they do not bother me.
Although I use my 28 Summicron and my 50 Summilux ASPH lenses more often, the 75 Summilux might be the last Leica lens I would consider selling. The 75 'lux is so unique: sharp, smooth, beautiful OOF. The 75 'cron is a newer design and an incredible performer, but if I ever purchase one, I seriously doubt I would sell the Summilux.