These are all regarding the D50; I can't imagine that the M10 is any different (except for a longer battery life).
Sony has better battery life I gather? not that the r9 sucks.
Battery life on the D50 is around 20 hours with four alkaline AA batteries, screen turned off, and mic-in power off. I have heard that the M10 pushes this up to something ridiculous like 42 hours or something on two AA batteries. Battery life for any of the Sony units is not a problem.
how are the meters, is there a peak feature (numeric) as well as the continuous bounce ?
r9 meters......, edirol junk, IMO. hate them. useful, but hate them.
I really, really like the Sony meters. It is a continuous bounce for the bar metering with a peak numerical value. The amber light also makes them really easy to see. As an added bonus, if you feel like turning off the backlight to save battery life, the Sony also has LED meters at -12 and 0 so you can see if you are going over. You can also turn off the LED meters if you are in super-stealth situations or the like.
Sony has 4gb (4hr of 24/44.1) built in and takes micro SD, which I have a few 2 and 4gb chips around. will that just write over to the extra media once the internal has filled ?
the SD card of the R9 is handy. I have lots of those already that are proven reliable in the Edirol.
The internal 4GB of Sony memory will NOT write over to the external memory chip, nor vice-versa. This is an inconvenience. I bought a 16GB Sony memory stick that gives me 8 hours of 24/96 recording. That was enough for me to capture the entire Scion rock fest in Atlanta this year, which is the gold standard of field recording for me. If you get a large memory stick / card, you will probably be set for your normal taping needs; if I attended Bonnaroo or a multi-day festival, I would probably buy an extra memory chip or something.
are their "sounds" pretty much hard to differentiate? that will make my choice easy, based on feature sets if the sound of the decks is very much on par. If one in stand out in all regards, mic and line in..., that's another story.
I don't know anybody who has A/B'ed the Edirol and Sony units so I can't answer that one. As Leonard (guysonic) measured and from some other online sites, the Sony D50 has better specs than the Edirol, but that doesn't mean a lot. My guess is that they are fairly close. I have no idea about the M10 vs. the R-09HR.
What ultimately made me chose the D50 over the R-09HR was the fact that all of the external controls are on the outside of the unit so I am not wading through an endless variety of menus. The M10 has a similar layout in this regard. The D50 also has four separate circuit boards to handle audio duties. I am not sure if the M10 has this level of over-engineering but I know that the Edirol does not. Finally, the D50 (and the M10) has a very useful limiter function where the unit runs two separate A/D converters, one -12 db from the other so that if you clip, the unit immediately switches to the second one to capture the unclipped signal. Very, very useful in stealth situations or indeed in any situation where you are away from your gear or on a bathroom break or something.
Sorry to seem like I am fluffing the Sony so much, but I really like the D50 a lot. The amount of care and attention to detail that Sony put into the unit is evident and this deserves to be supported. I have had mine for a year now and have not experienced any problems at all with the unit after using it in a variety of taping situations (stealth, open, festivals, etc.). Each unit has their adherents and I am sure that you will find lots of Edirol people saying the same things about their stuff as well.