Personally I am a fan of the Welz set from a collectability standpoint. Elliot Welz who is also known for his near perfect Lionel restoration work made these sets and similar Amfleet cars in the 1980's. They are not as refined as the Williams sets, but they the just have a nice feel and the pantographs are much more realistic.
The Williams sets are certainly better runners with dual vertical can motors in the lead unit. Mine are from just before the Bachmann purchase and have the Trueblast II sounds. Generic 5 chime horn with the long-long-short-long grade crossing pattern and bell. The only caveat is that they all use the same frame as the power unit so they don't take to grades well.