Another ME50 fan here, and markb's summary comparison of the ME's is :
I'd just add that IMO, the ME70 cut out about 1/2 the OD/Dist models and many of the other modulation fx the ME50 offers, sticking mainly to its Boss units, dropping competitors' units. But the ME70's 5-band EQ mode is a big plus vs. the ME50's limited choice of preset EQ's of its 'Tone Modify' function.
I've never had an RP unit, just tried them in the store, but I'd had a Digitech Genesis I, a little tabletop amb/cab sim unit with some mod & delay fx, a precursor to their GenX units. Once I decided the only amp 'models' I needed to get what I want were Fender Tweeds or Blackface models, or Vox AC30's, since hi-gain Rectumfryer amps are useless to me, I dumped the aggravation and signal loss of my collection of pedals/pedalboard for the ME50. The pedal-like knob interface vs. scrolling and hunting through cryptic LED or LCD menus is, to me, what seals it.
^^
AXES: Fender '81 The STRAT, '12 Standard Tele, '78 Musicmaster Bass, '13 CN-240SCE Thinline; Rickenbacker '82 360-12BWB; Epiphone '05 Casino, '08 John Lennon EJ-160E; Guild '70 D-40NT; Ovation '99 Celebrity CS-257; Yamaha '96 FG411CE-12; Washburn '05 M6SW Mando, '08 Oscar Schmidt OU250Bell Uke; Johnson '96 JR-200-SB Squareneck Reso; Hofner '07 Icon B-Bass; Ibanez '12 AR-325. AMPS: Tech 21 Trademark 10; Peavey ValveKing Royal 8; Fender Acoustonic 90, Passport Mini, Mini Tonemaster; Marshall MS-2 Micro Stack; Behringer BX-108 Thunderbird; Tom Scholz Rockman. PEDALS/FX: Boss ME-50; Yamaha EMP100; Stage DE-1; Samson C-Com 16 L.R. Baggs ParaAcoustic D.I; MXR EQ-10.