They're not *epically* different. Image-wise, you shouldn't notice anything different at all about them, they're optically identical. I've owned both versions--my first one was a single-scale which I bought almost silly cheap because it had some haze and a sticky aperture ring. Both of which the legendary Malcolm Taylor sorted out for me.
I eventually traded it in as part of a payment on the Summicron version IV. However, I missed it so badly--yeah, the version IV *is* probably a better lens, but the Rigid just has that charisma, primarily in feel and use rather than images--that when my local second hand dealer had a beautiful dual-scale Rigid come in, I bought that from him, too.
Now don't quote me on this, because I very could possibly be wrong, but I think the earlier version was made entirely out of brass. The focusing ring on the dual-scale model is, I'm sure, aluminium. So in some ways, because of that, because it was the first Rigid version and because I kind of fancy the earlier focusing ring's looks a little better than the later one, I'd give the edge--but only by a whisker--to the aesthetics of the single-scale lens.
However, my own dual-scale lens is indeed a beauty made to the highest standard, and a joy to use.
Not only that, it may be (I've read various sources on this) that the later lenses have slightly more scratch-resistant front coatings, and may be be more commonly found with less haze than the earlier lenses. The early lenses were notorious for having very soft and easily scratched coatings, and whether it was from the lubricants used in the lens or the reaction of moisture with those coatings, haze was a frequent problem. My dual-scale is clear as spring water, and I've not seen a dual-scale (in the few samples I've looked through with a flashlight) with bad haze.
Main thing is, find one that focuses smoothly, that has a properly working aperture ring, and which is free from haze and cleaning marks/scratches. If you do that, it's going to produce great images. The differences in feel/use will be negligible, and I'd have to believe you'd be quite happy with either one.