They should fit the same. Both 511 last. Going down a half size and up one width will probably work for you assuming you're not cramped in the 9.5C now. I could use a bit more room in front of the small toes on my 7 2E Dundees. I tried going to a 7.5 2E on my Daltons they had too much width in the ball and heel areas. The heel seemed almost as wide as my old 7 3E Dundees. I'm exchanging the Daltons for 7.5 E.
Hmmm. Not sure we are on the same page here... Or perhaps I just need lunch (and more coffee).
FYI - I don't think they make C widths in Dalton/Dundee. I was just using 9.25C to say what my ideal fit would be in the Daltons. I'm in a 9D right now, which is slightly short for me now, with some heel slip. 8.5E was way too short, 9.5D had too much volume, 9.5B was too narrow, don't recall if I tried the 10B or not.
You mention they should fit the same, but then say you wear different sizes in each? Based upon your examples (7.5 E Daltons and maybe ideal Dundee would be 7 and 2.5 Es?), it seems the Dundee fits longer and more narrow?