Both Polyshades and Varathane are light-duty finishes that won't wear well; that's how I read the 'not for floors' warning. Because a threshold sits above floor level it's prone to more scuffing of shoes than any other part of the floor (except, perhaps, the nosing of stair treads), and should have the toughest possible finish. I would stain yours to the appropriate color - Minwax stains are widely available and can work fine, but others are good too. I don't use a lot of stain in my work, but I've had good luck with Sherwin Williams'. Anyhow, once the stain has dried I would apply three coats of polyurethane, allowing each to dry and sanding lightly between coats.

Thanks jon1270! Thanks for the point about thresholds being more prone to scuffing--that hadn't occurred to me.

Yeah, I spoke to the guy at our local Benjamin-Moore (we don't have a Sherwin Williams close by). He confirmed what you said about the Polyshades being a light-duty finish that doesn't wear well. He said that if I wanted the risk of scuffs, I could put more coats of polyurethane over a Polyshades layer, but it would be better to start with a stain.

Thanks again!
la_koala