the RX has both magnetic and vibration methods of sensing a bite/run also has a built in transmitter to a pager in your pocket ,also has range testing facility and I would say this is the top of the range unit
the MMX is a superb little alarm with volume and tone controls and tells you a normal run or drop back
so it depends on your needs and of course how much you want to spend
to be honest its hard to find a "bad" alarm these days
I'd go forDelkim TXI's which provide superb bite detection, are utterly reliable and will run on a battery for literally years. The last timeI changedthe batteries was whenall four of mine blew into Wilstone Reservoir two years ago and the batteries shorted.Despite being fully submerged for 15 minutes they all stillwork perfectly.