I haven't used either the X3 or Fiio so couldn't definitively say, there's nothing exceptional about the S5 other than value though, as you say it can be had considerably cheaper than the X2 and much less than X3 or Fiio players and the bundled phones are nice. It's not going to win any awards for sound or UI but likewise it won't disappoint for the money either.
I bought it cause I can't afford a Shanling. :( I hope the Benjie S5 does me okay. I also have a Walnut v2 on order though, but that's not going to be a daily player or will it?