Federal banking regs consider 1-4's to be residential properties, and 5+ units to be 'commercial'. The underlying reason being that up until 4 units, there is some possibility of owner occupancy being the primary motivation for buyers, and income might be the secondary motivation. With 5 units and up they figure that it's all about the income, even if the owner lives onsite.
Ergo, the Small Residential Income Property (SRIP) with its funky adjustment grid is intended for use in appraising 2-4's, and the 71b (4-page) with no adjustment grid for the 5+ properties, theoretically with a $750,000 transaction limitation. Obviously, the 71b is so old that it isn't very USPAP compliant without quite a bit of extra help. I know at least a couple appraisers who use the 71a form (8 pages) for all 5+ units because they consider it easier to do a good job using that form. Personally, I only do a few 71a forms a year because after a certain point (about $1.5 mil) my clients want narratives. I use the 71b quite a bit. Matter of fact, I just finished one today and I expect to finish another one tomorrow.
IMO, it's eventually less work for the appraiser if they use the format that comes closest to being geared toward the assignment at hand. Regardless, it's the appraiser who makes the appraisal; not the form.
George Hatch