I'm using the Schneider 6x loupe (big one, for looking at 6x6 in all its glory). I also find I love it as my focusing loupe for my Chamonix 045N2. To me it is worth the investment, but it is pricey, at least in China. I got mine in Beijing at the Wukesong market, at the large format camera store.
I also have a cheapo 10x loupe for when I want to look very closely. And if I really want to get close I have a 50x loupe style microscope... ;-)
Oh, and one more thing: are your negatives sharper than your scans? Yes. Don't even need a loupe for that, I think I'd bet that whatever scanner you use, the negative will still be sharper.
I dont know how sharp my negatives are, that's why I want a loupe. I've never looked at them closely so can't tell. My eyes need a lot of help seeing detail so small.