Some of the buses in Asia (and some in Europe) tend to not be as disgusting as US buses. It appears "megabus" is now in some US markets and is a slight upgrade to Greyhound.

I'd still rather be in a private car and just pull over whenever I want to use a bathroom, or get food, etc. I wouldn't feel particularly comfortable sleeping (particularly with my luggage) on a public bus in the US.

I'd also pay extra to have a machine drive me from SF-SEA rather than a human driver, as I'm somewhat antisocial and dislike being around random people like taxi drivers in car-sized spaces for that long.