A big thing for me is the display. I know color displays may be viewed as unnecessary, but the display on the MRX beats the hell out of the dirty yellow display from the Lifepak's. It takes nearly no time to recognize the waveform or data point you are looking for whereas you have to double check what you're looking at on the Lifepak due to the mono color display.
The MRX connectors also stay attached to the monitor much better. I've had specific issues with the Lifepak's ECG cable coming unplugged. Maybe it was just worn out. Also the ECG cables on the MRX are much longer than on the Lifepak. The 12-lead plug is better on the Lifepak, however because of a better connection and the fact that it doesn't start out as a 3 lead like the MRX does.
As far as controls, I don't think one is really superior to the other. The MRX has a quicker control for energy settings, but until you learn the monitor well, you can spend a lot of time looking for a specific function. The Lifepak has a simpler menu and I believe more options for descriptions for manual time stamps. The dial for the options works well as long as the ride isn't bumpy. Very easy to press the button/dial in by accident. The buttons on the Lifepak could use a little more tactile response but I believe that is a longevity issue. I think a good setup would be a combination of the two, like two dials and about 6 buttons.
B/P and SPO2 monitoring are a crap shoot in either case. For anyone who I am really interested in watching their SPO2, I use the disposable sensors for either monitor. B/P, well, they're both machines. Take the reading with a few grains of salt unless the patient is completely still. Then only a couple grains of salt are necessary.
ETCO2 monitoring is something I haven't had to do very often with the Lifepak as I haven't done a vent transport with a Lifepak. Usually the pt. is newly intubated (within minutes) so the capnography isn't being utilized through the monitor. The MRX has been very dependable on capnography, but I don't really have an accurate comparison between the two monitors.
The MRX is nicer for charging as you don't need to bring the charging center with you if you are changing rigs. With the MRX you only have to bring the basic cord.
I don't think either monitor has a decent pouch system. It's a minor complaint, but it is a daily issue.
That's all the complaining I can think of.