It's likely that the quality is jumping significantly from the 6" to the 8". Many of the really cheap brands are either not well made, or not made to be durable, or not made for "fine" work. They'll likely do for sharpening things that don't need to be perfectly flat, but may not work well for, say, a paring chisel, where it really does matter if the edge is straight.
Personally, I'd recommend either DMT (which I use, and have been quite pleased with) or Eze-Lap, which are used by at least a few people I trust to get things right. Neither brand is what you'd call cheap, with 2"x6" stones running from $25-39, depending on manufacturer, coarseness, and current sales. Something like this set
is a pretty good starting place, although I now kind of wish I'd started with the 8" plates. A DMT Duo-Sharp 8" with fine and extra fine on their base, plus an extra-coarse/coarse plate without the base would probably be a really good way to go. That set should handle nearly everything, and comes in around $130 on Amazon at the moment. I'm sure Eze-Lap has similar setups, but I don't have those bookmarked.