I have to admit Alex, that I went with the Marantz PMD620 over the M-Audio Microtrack II based purely on the brand name. Marantz have also been in the portable recorder business much longer than M-Audio.
I'll only be using the line inputs of the unit, I hear the mic pres on most of these budget recorders are average. You have to spend a lot more money to get decent pres on a portable recorder. I'll be using the PMD620 with my PSC Alphamix portable field mixer.
