Really the only advantage of Legacy vs Electric RR Cruise with Railsounds is the ability to add more smoke effects. Slow speed is 100 steps vs 200 for Legacy. Hardly noticable.
PS3 offers the advantage of not having to deal with making an antenna but you should add pickup rollers to the tender as many early 3rd Rail did not have them there. Both ERR kits and in particular PS3 kits come with most of what you need. If you tried to install Legacy you would have make your own harnesses to connect the components.
Pete