Both feature direct conversion receivers with I/Q output, but the QCX has analog active filtering, while the QMX is an powerful SDR.? A multi-band QCX could be great on a small battery pack..
According to the web page a QCX-mini has 72mA current consumption at 12V and a QMX has 80mA. So there's hardly any difference. And in fact QCX-mini would also have to increase its RX current if there were band-switching mechanisms too (even if there was space for such a thing inside it). TX current of the QCX-mini is better than QMX because QCX-mini is Class-E vs push-pull Class-D in QMX. But considering there would be 50-60mA extra for PIN diode filter switching on TX (which is much less on QMX due to the buck converter)... it's not really practical on a QCX-mini nor beneficial, for many reasons.?