4" in my house. Old cast iron was 4 and I replaced it with 4" ABS.
There is a difference in efficiency of flushing according to the plumbers at my school. I installed a low flush about 12 yrs ago. It's ok but not spectacular. One problem is that the water in bowl has too small an area so that it needs frequent cleaning.
The only way to test and compare is to line up all the options, drop some large loads and see how each deals with it. I don't know if Consumer Reports has done this.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13