Mine are on the semi-dress last, but nevertheless they seem much roomier than the SDs that I have tried on in the past. My sizing attempts have been complicated with White's. (Though I admit that I seem to have weird, hard to fit feet... I tend to measure on the Brannock between 9.5 and 10D, but I always have needed a slightly wider boot I know, and I only recently found out from the foot doctor that it's because i have both high arches and very flexible feet, so that my foot really spreads out when I put weight on it.)
Anyway, about a year and a half ago I had a chance to try on a pair of SDs in 9.5D, and they felt really tight in the toes. Some time later I sent tracings to White's, who said I should get 9.5Ds, which I knew would not work. So after talking with Kyle, he sent me to try on a big pair of SJs (like 10" with big lug heels) on the SJ last in 9.5E. These too felt surprisingly tight to me in the toebox, but the length seemed right, so Kyle said let's go with 9.5EE in the SD last. But the ones I have now (SJs built on SD last = "bounty hunters"), seem MUCH roomier in every respect than the regular SJs I tried on, as well as the regular 9.5D SDs I had tried on. Length seems quite a bit longer than I anticipated, but also there is just a lot room in the toe area height wise. They are probably actually too big for me, but I put a pair of Superfeet insoles in and I think that will work.
All of which is to say, I have found sizing to be pretty unpredictable and hard to pinpoint.
FEELwise, they just clearly feel bulkier and more workboot like than the SDs. When you look at them side by side they don't look like they would -- just an inch higher, etc, but you can really feel the difference when you wear them. Partially no doubt because of the full bellows tongue, but I can't remember maybe the SJs have a steel shank...? Not sure about that... Mine are also fully lined, so that would contribute. I know there will be some ankle rubbing during the breaking in, but I think otherwise they should be not too bad in that regard.