Can't say I'm an expert...
But I do own both of those models.
I think they're both top-notch!
They are great bargains in the new locomotive universe. The RS3's like their GP7's can be a little noisier than those put out by Atlas, but they start at a low speed, have decent pulling power and the detailing for the price of the locomotive is acceptable. The couplers Bachmann uses doesn't thrill me but they can be changed easily enough.
I'm also not big on the electrical pickup system Bachmann uses, which are essentially etched brass "wings" that have to stay in contact with the body/frame in order to ensure contact. But again, for the price of the locomotive, including decoder, I can live with it.
The Bachmann S4 quite frankly is the best operating N-scale switcher I've operated. Smooth, quiet, creeps at slow speeds, maintains good electrical pickup through un-powered frogs... I can't recommend it enough. I do not have the Atlas MP15, but I do have 2 VO1000's, a Kato NW2, and three different Lifelike/Walthers SW800/1200– and the Bachmann is a quieter/smoother/better runner than any of them. And for the price... maybe the best locomotive bargain in many years.
