Having heard both the ESU Loksound decoder and the Soundtraxx Tsunami in both Athearn Big Boys and Challengers, I will say that the Soundtraxx decoder is superior as far as sound goes, particularly rod clank and the whistle...with the ESU's whistle sounding NOTHING like a prototype FEF/Challenger/Big Boy chime (they were all the same). The equalizer feature in the Soundtraxx decoder allows me to fiddle with the frequencies I can hear coming out of the small speakers enclosed in airtight enclosures, and I think it makes it possible to get a much more pleasing chuff and hiss when the speaker is not being bombarded with frequencies it can't physically handle.
Although people swear that the ESU offers finer overall speed control...for mainline running, slow running through yards, no switching...I prefer the better sound, since the control differences are minuscule.
However, for diesels...it's ESU all the way!
Cheerio!
Bob Gilmore