What is your reason for wanting to visit either? If it's solely for the purpose of visiting a President's home, then Mount Vernon would win out based on the drive time. Going to Charlottesville to visit Jefferson's estate will add a couple of hours to your drive time; it isn't really "enroute" to Williamsburg.
Both are interesting to visit. The museum at Mt. Vernon is quite good, as is the movie that is shown in the Visitors Center. There is much to do on the estate. George Washington considered himself to be a farmer, and the farm has a lot to see and do. At Monticello, they too have a new Visitors Center and museum. The house reflects Jefferson's interest in architecture (did you know Jefferson designed the state capitol building in Richmond?) and the estate still has some of the gardens as designed by Jefferson.
So, excluding drive time, it's really a toss-up. If neither holds a stronger interest for you and your traveling companion(s), you may as well toss a coin. You won't be disappointed either way.
I also wonder about your single night to Williamsburg. There is a lot more to see than a day will allow.
I have visited both Mount Vernon and Monticello and it would be hard to recommend one over the other. I agree with a previous poster though that it makes more sense given your short amount of time to visit Mt. Vernon as you would be going out of your way to visit Monticello.
With all the travel you'll get an overview of either place...as well as Williamsburg imo. But that's okay too. Come back for more...or stay longer.
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