I guess it depends on the bus; I've seen some fairly luxurious long-haul buses. In general though, train travel is a lot more comfortable. I'm a pretty tall guy, and bus seats (never mind plane seats!) are rarely spaced out enough for me to sit comfortably - never a problem on trains.

On long bus rides you will typically have stops along the route to stretch your legs, get something to eat etc. Sleeping in a bus seat is no fun though, so I would avoid overnight trips.