M.L.
Oh! You have to choose? That's terrible! I love them both! But if you absolutely have to choose, you might pick Monticello because Thomas Jefferson was such a multi-faceted person and it shows in his property.
But I say that almost with tears, because Mount Vernon is marvelous. You'll miss the formal parlor with its colorful - is it wallpaper or is it paint? - the beautiful gardens, the farm, the VIEWS...
Both are great places to visit, and both are better than they were thirty years ago.