roller with enough stain to make the application worthwhile. Stain doesn't hold onto a roller like paint will. IMO you'll be much more efficient with a sprayer and a backroll, unless by efficiency you mean getting the least amount of stain on the wood as possible.

I don't mind getting more stain/protection on a fence I'm trying to protect than less. The goal is for it to soak in, not sit on the surface like paint.

Pump sprayers are very inexpensive, toss it after you're done.