I find the simpson last is a narrower fit, although it is definitely a longer last. The Inca, from what I can tell, is not elongated and is more generous in width. My foot measures an American 10.5D. I tried the 9.5 in the Inca double monk and I found it slipping in the heel way too much even when I tightened it. I went down half a size and there was no slip at all, and maybe it is just a touch snug with a thin dress sock. My toe still has about an inch left of breathing room.
In other news, I also have a simpson lasted penny loafer which feels ok with socks but a little too big without them. If your looking at a loafer it's probably best to go down half a size if you plan to go sockless.
All in all, I highly recommend trying these shoes on if you can. I've learned my expensive lesson already by ordering multiple pairs of Alfred Sargent and Crockett and Jones, all of which ended up being too large or small.