sounds aside (too subjective), the specs differ some. korg m50 is newer than juno-g, so, expect more.
m50 > juno-g:
wave ROM 256mb vs 64mb
5 insert efx, 170 types vs 3 insert efx, 78 types
2 arpeggiators vs 1
pattern/linear recording vs linear only
bigger touchscreen vs smaller screen
juno-g > m50:
sampling, audio recording, although you can load and play back samples with the m50
bottom line, if you don't need to do sampling or audio recording, m50 definitely has more power, but, if you do, cross it off the list.
neither one has aftertouch