Yes, brand matters. A cheap filter will degrade your IQ. I'd recommend B+W or the higher end Hoya models (S-HMC, Pro1, HD).
Personally, I find a 3-stop filter to be ideal for most uses. Consider that you might want to reduce reflections and enhance skies by stacking a circular polarizer on the ND filter, and the CPL adds another ~1.75 stops of density, so the total is close to 5-stops which is plenty for a long enough exposure. I also have a 10-stop ND, too long for waterfalls, that one is great for architectural shooting - a 30s or 1 min exposure is enough to blur out pedestrians and other passers-by.