Pepa,i think in real terms you won't find a lot of difference between the two detectors regarding depth.I use the M6 and it will keep up with most machines if you set it up correctly,which is very simple by the way.Your choice will boil down to the features,simplicity of operation,ruggedness etc,etc.I owned an mxt and the M6 was just as deep.......I don't believe that the mx5 is deeper than an mxt.
I prefer the M6 out of personal choice and i can't say that it's better than the mx5 because i've never used the mx5.I like the M6 because it has excellent all round performance both inland and on the beach.It has knobs and switches which I prefer(makes the detector feel better quality to me) and is the simplest detector to set up i've ever used.
The mx5 looks to be a good machine from what I have read and if it performs as well as the M6 you're going to be happy with it......i think you are on to a winner either way.