well, you're options are basically the M-Audio Microtrack II, the Marantz PMD-661, and the Sony D50.
I don't like the internal battery of the Microtrack II, and I had a bad feeling about M-Audio in general after using the original Microtrack. Other people have had good experiences with the Microtrack II, but in my opinion, it's not a rock-solid recording device.
The Sony D50's digital input is via optical, which I find is not too convenient in the field, especially if you're A/D converter doesn't have optical out and you need to deal with a coax to optical format converter. one more box to carry around, and it's a hassle. FWIW, the AD2K does not have an optical output.
The Marantz PMD-661 is a great deck, with a coax S/PDIF input. and in my experience over the past 7 or 8 months, has been rock solid.
for a little more money then the three decks listed above, you've also got the following options:
Marantz PMD-671, Tascam HD-P2, and the Edirol R-44.
All of these are also larger decks, but have more features. for example, the HD-P2 has word clock ins/outs, so it's very easy to sync to other decks. and the R-44 is a 4 channel recorder, so that also opens up a lot of options.