Hey Marnold, good question!

Well I like them both for different reasons.

1. When I play a gig/jam and want to have a flexible set up I can change on the fly, I want the M9. The reason is because I can instantly adjust the knobs on the unit in the middle of a song, if need be. The HD500 takes more work to edit on the fly.

2. The HD500 is an audio interface too, so I can use it straight into my computer. It also has amp modeling, and that HD technology, whereas the M9 is just a stomp box modeling.

The HD units are "better" in terms of features, but I still prefer the M9 when I go guitar - stompboxes - tube amp, because I don't need or want amp modeling in front of my amp, and I want to be able to quickly adjust an effects parameter at any time.

Now, if I were to play in a cover band where I need specific tones for each song, the HD500 is perfect. I can run it into the power amp section of one of my tube amps, and get a killer set up. However, I usually want simple and generic tones, and a quick set up, which is why the M9 is a definite keeper for me.