I have both a Weaver one (First edition with the tower chain drive) and an MTH one. Living near Schenectady the former home of Alco, i also have 3 PA's (AA,AA,AB), 1 FA (AA), and 1 RS1.
Back to the RS3's -- the Weaver one is the most accurate and true to scale, and if it's an early one (fixed pilot) with new gears on the drive (from P&D) it's a great runner at nice slow speeds, AND can often be found at train shows or on eBay for $60 - $100. Newer issue Weaver RS3's have vertical motors (China Drive) and swiveling pilots if that's your thing.
The MTH ones IMO are of better quality than either the Lionel, WBB or K-Line ones and next to Weaver are the most accurate ones.
I've added TMCC to both of mine, and RailSounds to the MTH one; at the time i did the Weaver one (over a dozen or so years ago) there wasn't enough room in the body for a sound board and speaker, and no room in the fuel tank for the speaker with the driveshaft running through it.
Hope this helps.