First you have to ask yourself do you have room and want to mess with the rear speakers in the 5.1 set up. Unless they are wireless then you won't have to worry about the wires all over the place. I have a 5.1 system and after trying to hid wires and or tripping ovber them or my wife complaining about the wires i just took the back pair off. You situation may differ though.
okay, one of the units you chose was 2.1, but it included a separate box instead of integrating into one of the speakers. Unless the audio quality is better, this seems like added CLUTTER you can avoid.
I'd look up several reviews for each model, but my gut tells me the Z533 is probably as good or better than that 2.1 unit from Genius.