I've not played the ME70, but have played the 50. Just like it says.........super easy to use, no programming, just turn and play. And while I didnt love all the patches, I did find what I needed out of each section (drive, mod, delay, etc.)
I like how the 70 has another pedal for comp/booster and I love the fact that it has a looper.
I have played both, own and gig with the GSP1101 that the RP500 is based upon and, IMHO, the Digitech RP500 has a warmer, slightly more midrangy classic analog distortion tone without that signature Boss metallic sheen.
If you like the super bright Boss metal tone, go with the ME70, as both are great units.