II have 10 of the MTH SD70ACe units -- Demonstrator 1 (hi-rail/proto-2), Caterpillar 1201 demonstrator (scale/proto-3), Bush 4141 (scale/proto-3), ATSF "Isle of Denial" (scale/proto-2), UP/MKT (scale/proto-3), UP/CNW (scale/proto-2), UP/WP (scale/proto-2), UP/DRGW (scale/proto-2), UP/SP (scale/proto-2), and UP/MP (scale/proto-2). They've all had great sound, especially the start-up sequence. They all run well individually and when consisted with other locomotives. One of our club members has a couple of Lionel TMCC/Legacy UP Heritage Units and they also sound great, but the start-up sequence isn't as good. One thing that I noticed with the Lionel units is that they like using dual speakers which produce better sound quality. I don't know if they're stereo, series or parallel, but they sound great. This is one place where I think MTH seems to have cut corners -- the speaker. I've acquired several full-range speakers I plan to test out which I plan to use as replacements.
As to which locomotive to buy, that's just a matter of which command system you like and which locomotive offering you're after.