I do not understand the last line:
“I am not sure how good is the sound from it and the model is a little bit old”
MC6000 is as I recon not a bit older than S4 but the other way around.
The sound in MC6000 is considdered the best you can get in a controller!
Also the MC6000 has better quality in chassie, buttons and knobs than the other two.
The only con is that you have to upgrade the LE to pro if you are in EU or buy a pro if you are in US.
I would go for the MC6000 if I were you!
But you should go to a store and compare the sound and quality yourself IMO!