I have never ridden on a greyhound..But last year and a few other times I have ridden amtrak.. As far as price goes.. you CAN'T beat it. $327 got me from Lincoln, NE to Emeryville, CA and back. certainly would not have been able to drive for that price:)
Things to consider (and I would imagine this would be WORSE on a bus): amtrak has 24 hour car attendants that keep stuff clean.. the bathrooms are still icky.. And being stuck in one of those seats for more that a 24 hour period SUCKS..and you have to hope you don't get stuck next to some nasty that has his eye on your stuff, or wants to talk your ear off.. or wants to sleep on you.. Maybe on a bus its different, but usually on amtrak you get assigned seating.
One more thing to consider- you have to know your chit to become a train conductor.. A bus driver?? Not so much... They can and do hire Anybody- hell they even hired my best friends father, and he is NUTTIER THAN A PET COON.. how he didn't cause an accident at his several month long stint as a GH driver, I will never know..
I say IF you can afford to and have a good car.. DRIVE.