IMO it doesn't matter that much for a couple of reasons. If you want to be very accurate with your measurement, you would need to determine the refractometer's correction factor, even if it was dual scale, because there is always a blend of sugars in the wort, which would throw the reading off (even if only slightly). (This is done by comparing refractometer readings to hydrometer measurements. The different sugars do have different refraction indicies.)
I use only a refractometer for pre-fermentation measurements. I'm not terribly concerned with absolute precision with regard to specific gravity, because other than determining exact efficiencies and alcohol content, such precision (IMO) is not terribly useful. What I focus on is consistency. This is where a refractometer shines, because it allows you to take many quick measurements throughout the process. If your last pale ale came out good and you know the first runnings were x, the last runnings were y, your preboil gravity was z.... Then when you make it again, you shoot for the same measurements. The fact that the Brix or Plato measurement may not translate perfectly to specific gravity has little relevance.
Having said all of that, if they are the same price, you might as well get the one with the SG scale, because at least that way you won't have to deal with converting to SG or getting used to working in a Brix/Plato scale. If a Brix-only unit is cheaper, that is what I would (and did) go with.
