as you're probably noticing, when you're dealing with anything mid-range or higher -- especially with cartridge bearing hubs -- most hubs have pretty solid bearings. the main differences between hubs are really going to be in strength, weight, and aesthetics. nicer hubs will have better cog and lock ring threads, and will be stronger (better turning, cnc, etc) and/or lighter than their counterparts. if your primary concern is rolling resistance, you could really just put ceramic bearings in a formula hub.