I use Keyshot 6 as my renderer, and I have found that the best format for importing into it is STEP, which also preserves the vectors, but seems to load many times faster than 3dm. I’ve never actually loaded a 3dm file fully into KS as it takes so long and nothing seems to happen, so I can’t verify if it works.

Obj files load quickly, as there are fewer calculations needed, but I find that the artifacts of having made surfaces into polygons are often very visible in KS shadow calculations or transparent materials, even in high-poly files, so STEP is far better if you have it available. It always looks smooth and exactly as intended, save for some occasional glitches I have encountered, especially when working on complex surface objects that have been lofted and then twisted. The same glitches have shown up in the Obj exports, too, so I don’t think that’s a STEP issue, but rather some of the data from MoI being misinterpreted by KS.