The M3 is powerful enough to decode virtually all video up to 1080p. It's 1080p max though, no 4K.
Drawbacks of the M3 vs Q5 are that it's less powerful (particularly noticeable for games), weaker wifi (no antenna), fewer ports and a slightly strange design.
The M3 is a great box for playing video at up to 1080p. The Q5 is a more versatile all-round machine.
For reference, Blu-Ray rips are usually below 45 Mbps.
Using the MKV sample files here http://jell.yfish.us/, played at 1080p:
35 Mbps - M3 OK - Q5 OK
70 Mbps - M3 OK - Q5 OK
110 Mps - M3 OK - Q5 OK