This topic is one I have found very hard to find reliable and detailed information on.
Also recording means different things to different people.
I am partial to FM recording specifically lectures and recordings
for podcasting of human voice as opposed to microphone or
LINE IN recordings.
The best recorder I have found is the Sansa Fuze+
Bear with me while I explain what I mean:
Regardless of Rockbox's support for this unit's recording
capabilities, it is easy to dual boot into the original firmware,
record FM or mic, and then reboot into Rockbox for playback.
The original firmware has the best FM tuner and ADC
hardware I have encountered. Also the user interface
allows saving and restarting the recording process
during the applause between speakers. The resulting
recordings are WAV files at 1411 kbps and in stereo.
That means raw 44.1 KHz sampled 16-bit dual channel
WAV files. Same as CD blue book specs, not lossy
ADPCM style compression.
The older Sansa e280 OF produced 512kbps FM recordings
also in WAV format but not at Blue Book quality.
Do you want your recordings in OGG, FLAC, MP3, AAC,
Speex, Opus, or WavPack? Dont make Rockbox
record it in that format. Doing that cuts down on your
Recording Battery Life. Instead, maximize the duration
of your FM recording time: Boot into the original
Sansa firmware, record in wav format and transcode
it later using the quality settings you really want.
All the command line encoders accept WAV input.
Make the audio file formats that you are comfortable
with, but start with high quality input.
And that means Sansa Fuze+ Plus
PS: I get about 7-7.5 hours of FM recording time on a full charge, so you need to start with at least 5GB free. Thus I do not recommend buying the 4GB Fuze+ unit as the battery will last longer than the memory
and you cannot record directly to an SD card.
I would be interested to learn if LINE-IN recording is possible with this device.