The really expensive Asco loupes are achromats, the next level down (140 bucks) are aplanatic. As I've always understood, achromats are usually also aplanatic, so they deliver accurate color and an undistorted view across the lens. Aplanatic might have some color error. The color gets important when dealing with precious stones but isn't an issue for us watchmakers. I have several of both, and honestly can't tell which is in my hand without looking at the loupe body to see.
In lower powers, the aplanatic issue is sort of a non-issue, as the simple single double convex lens delivers a good image. In higher powers it's almost a necessity that the loupe be a double lens design or there is awful distortion. Most good brands go to double lens groups (two single convex lenses) at around 10x. Seems like there used to be more offerings of double lens loupes in the 5x-ish range, I have a Wild Heerbrug 5x with double lenses and am pretty sure I have an old Asco or Bergeon or two lurking around here the same.
I think all the current offering from the Swiss suppliers are good, you can't go wrong with Asco/Bergeon/Horotec.