First of all, for the money these are both great units. I had an M-5 and an HD500 and was looking to get smaller for my acoustic gig, so I bought an M-9 to replace both. Size and function for the M-9 are great and it functionally is a great replacement for me.
My beef with the M-9 and M-5 is as follows.
I purchased the M-9 used (E-Bay) and the power supply was DOA.
The power supply is huge (bigger than the 3 amp power supply for my HD500) I bought Velcro to secure the new transformer to my pedalboard.
My real beef is that Line 6 utilizes midi connections to update firmware. Not only that, but in order to update the firmware, I am required to buy a specific midi cable from a third party in order to perform the update. Line 6 should take a lesson from it's competitors on firmware updates. Instructing customers to buy a third party's hardware is a testament to their support.
My opinion: Support your own product with your own hardware.