The 424mkIII is way beyond the MR8 in mixer power, for sure.
The 424mkIII has a 10 input mixer to work with, more if you count redundant connectors. The MR-8 has 2 or 3 inputs.
Also, the 424mkIII records 4 discrete tracks onto normal Type II cassettes, with a 90 minute cassette lasting 22.5 minutes, and almost unlimited shelf life of tapes. You could shelve any piece of work and get back to it years later, if you wish, in full 4-track format. Type II cassettes are low cost/high quality recording media.
The MR8 records to 'Smart Media' cards, with the largest flash memory card being only a short number of minutes, then you're obliged to mix down your work to PC, to clear the card and start again. The largest flash memory card out there is like $99/ea, appx, and it gives you some scant number of minutes of recorded data. I can't see getting a 10 pack, or a few dozen of these flash memory cards, like you would get cassettes.
The 424mkIII records 4-simul-max, and the MR-8 records 2-simul-max. That 2-simul-max recording can be limiting sometimes.
Built-in effects never thrilled me much, since I have all sorts of outboard effects gear, and what's more, I use effects very little, almost never as of late.
That's all I can think of for now, but I'll probably think of more later.
The MR-8's really not 'all that', but like JR#97 said earlier, if you're stuck on 'digital', then the Fostex FD-8 blows away the MR-8.
As analog goes, the 424mkIII is current king of Portastudios, and is quite a nice machine, most notably for it's mixer section. 4-tracks to cassette is still a fun, inexpensive format, which in general delivers nice hifi sound. It's simple to use, not all 'gadgety' like the newest digitals.
It's a no-brainer which one I'd get.
