Advice from a geologist/geochemist who has used a lot of different loupes (from very cheap to very expensive): Save yourself about $350 and get a Belomo 10X Triplet with 21mm lens for $30. I am far from a loupe expert, but I cannot tell the difference between using a Belomo and using a loupe that costs 10X as much. The Belomo has a beautifully crisp, clear and wide field of view which makes using it a breeze.
Take the money you saved and put it towards a nice used microscope. Because even a basic microscope provides a much better field of view and ease of use than the most expensive of loupes.