Likely a fair amount. Distos, depending on the model, can measure up to 650' - I think the FatMax tops out at about 40' (the current model at Stanley's website says 40' anyway). If that's true (comparins specs from the makers' websites) then we can rest assured that Leica's lasers are better than Stanley's.
A Disto can calculate an unknown wall length based on the pythagorean theorem when necessary. Disto is also more (twice as) accurate: it is accurate to .01 feet (1/100 ft) - FatMax says it is accurate to .5% (half a percent). The Disto viewfinder, even on old ones like mine, are a Godsend on those 75' open-corner walls where you're shooting between rain gutter downspouts. I'd love to have the Disto A8 with the on-screen digital viewfinder deal...and would already have one if they weren't $800 and if my Disto were on its last legs, which does not appear to be the case.
As long as a Fat Max does what you want it to do, and is accurate enough for you, go for it. I still like my terribly old Disto though - it still gets the job done even if it is big and (6-7 years) old. I use it in combo with a tape (100' fiberglass) since the Disto is in a nice belt holster when I don't need it.
I check my Disto against my tape every so often, probably 5-6 times a year when I am measuring a wall or space that is easy and quickly done by both methods...My Disto and my tapes (I use up about 2 per year) are always spot on when the tapes are new, before they get stretched out and are less accurate. As I said, I can only get about 6 months out of a tape before it is too stretched/nicked/torn to continue using.
And there is a $129 Disto - the A2. For appraisers, I'd recommend any of the Leica Distos from A2 to A5 - the A6 and A8 are nice, but are really overkill for our needs. I do like the digital screen/viewfinder on the A8 though. For appraisers that *only* measure with a laser and never use a tape, the A8 might be just right for them.
