8,000 lbs minimun is the standard wisdom. 9,000 pound winches have become popular, as a marketing tool.
Everyone had an 8,000 lb in their lineup, so the mfg's that got 9,000 lb units at nearly the same $ picked up sales. Now, they all have 9,000 lb units.
Get the highest quality, largest capacity you can afford (in that order). The nice thing about increased capacity, is that even if you don't need more than 8,000, a 9,000 lb unit will draw less/run cooler than its 8,000 lb couterpart, when pulling the same weight.