How many channels do you want/need?
If you don't need more than two, the simple answer would be to get the fostex. But I suggest a more complex dual unit setup. Get a Sony M10 and a Sound devices preamp. That way you get a great small recorder to use with internal/pip mics, and with a SD preamp you get great preamps and monitoring. The M10 has awesome battery time and the internal mics aren't at all bad although the stereo spread is almost non-existant.
If you want more than two channels, I'd buy the tascam 680, add the low noise preamp mod when you can afford it.
I bought the R26. it's quite ok but have a few to many quirks in use, but it's a pretty affordable way to be able to record 4(6 with an extra hardware mod or using the internal mics) channels. Unfortunately the internal mics aren't good enough for low level sounds.