According to spec sheets, the Klipsch go louder. They have a smaller footprint and their tweeters are supposed to be better, if they really are the Tractrix tweeters.
The MX have a true 2.5 way crossover design, though, and the satellites are absolutely the right height for listening in nearfield - the Klipsch tilt the axis upward for the same effect.
I know that due to the short-sightedness of dealers you can't audition the speakers before you buy. I have the MX5021 and though they cloak the sound a teeny little bit, they're good for the money you pay. Reasonably accurate and smooth. I think the Klipsch will be a little more revealing and lively, as the typical 'Klipsch' sound is.
5.1 is good for positional audio, not so good for music, but then neither is a 2.1 system with the sub crossed over at 150 Hz. I can clearly hear the bass direction at those frequencies, and it's OK for a little boombox type of system, not so good for audiophilia. That applies to both the Klipsch and Logitech, BTW.
All the best.
This is an OLD thread, btw, try and keep the discussion on topic.
As I find most commercial equipment severely lacking in build and sound quality, I have departed from brands to building my own for myself as much as possible. I only now buy what I need, and then sit and upgrade to remove the limitations that manufacturers placed on themselves to keep costs down. This has led to modified Delta 66 and SB Live! cards, a home-built 100 w/ch amp (and more on the way), and Seas-driver based speakers which are being built at a bargain price. I will need to buy my Dynaudio monitors when I can though, but the monitor amps will again be handbuilt.