It strikes me that converting Williams to DCS or TMCC would cancel out the cost advantage for buying Williams vs MTH or Lionel locos.
Actually, I won a Railking F3 with PS2 a few years back on E-Bay for $105 so that's not always the case.
I have a WBB E7 that I bought as a dummy, as I wanted to repaint it (in Seaboard Air Line Mint Green) and use it for display. At the time all I did was strip the paint, replaced the molded-on grabs and such, and repaint/letter the body.
It looked so good I decided to power it, so I bought the power kit (no sound). Didn't like the way it ran (too fast) so I bought the ERR Cruise Commander kit (still no sound). Then I saw the MRC AC Sounder and added sound to it.
It was nice at that point, but that's when 3rd Rail offered their E7, ALREADY painted for Seaboard with TMCC (and sound) included. I looked at the price of it (around $600) and added up the cost of the WBB engine with all the stuff I added to it and realized I had spent almost as much as what the 3rd Rail engine cost (that's not counting all the labor involved).
I have since removed the electronics from the WBB E7 and put them in an old Railking RS3 and added the ERR Railsounds Commander to the Cruise Commander (the RS3 originally had Locosounds). The WBB E7 now sits gutted on a display case, just like I originally planned.
When you start looking the prices of these engines and comparing them against each other (and what you really want in a model), what Barry says is true...You get what you pay for.