It's a dilemma that's hard do decide on. I like to keep things as original as possible, that's why I spend time touching up the old finish rather than stripping it off and redoing it completely. I accept a few markes of age and wear, but at some point you might as well refinish the whole machine and get new decals. Decals can even be made. It's a lot of work too, you need to be happy with the result, because you don't do it for the money value. Some models like the Featherweight fetch good prices when refinished though, but it has to be done flawlessly.
In your case with the Rotary, if it doesn't say Florence or Worlds on the treadle base, you can choose more freely. I'm guessing both of these models came in much the same cabinet options, I and would go for the machine in best condition, or at least the one that restores to nice condition. I find it very hard to trash these old machines, particularly when they are in restorable condition. They are getting fewer every year, and there's still enough interest in them to take care of them, and let others have a chance at a fix up machine too.