Interesting perspectives so far. As Mr. Taylor points out, I already own a 152mm f/5.9, and like Don, the Vixen was the scope I cross shopped before buying the Astrotelescopes. I've used the Vixen a couple of times, so perhaps I had a bit more context, but the Vixens I've used were both in relatively poor collimation. My understanding is that Vixen has licked this issue in the more recent builds (these were units made several years ago; not the latest run).
The factor eliminated by the scenario is the price difference. New, the Astrotelescopes 152 was about $1000. The NA140SS is now no longer on sale and the new price runs about $2000.
Having used examples of both scopes, price aside, the Vixen is better made and better built. I agree that the SV25 focuser is a little better than the Vixen's unit. But the Vixen has an appropriately long dew shade, retractable or not; the Kunming unit's dew shade is woefully short. The Kunming is also quite heavy for its aperture, and extremely front heavy despite being a doublet. The Vixen is the easier mounter. The 12mm of additional aperture is a plus in favor of the Kunming, but here's the thing. The Kunming performs best at lower magnifications. Using lower magnifications reduces the limiting magnitude of a given aperture. To take advantage of the greater light grasp of the Kunming, you have to use magnifications that induce lots of false color on targets magnitude 2.5 or above. The NA140SS actually goes deeper before the false color becomes intrusive. Because the Vixen is a fast achromat and also yields false color, the flat field pitch isn't a selling point for me. The level of FC in the 6" f/5.9 doublet is much less than that in a standard SCT, by comparison. Really not much of an issue visually, and I wouldn't choose the Vixen for imaging (though some do).
For me these RFT achromats are really a niche scope; a nifty combination of a relatively wide, high contrast field with more light grasp than the typical extremely short focal length small aperture wide field scopes. So I bought based on price. I didn't want to make a huge investment in a limited-purpose scope. But were price not a factor, I'm not sure that I wouldn't have gone with the Vixen.