It pains me to say it, being a long time Apache Labs guy, but just looking at the spec. sheets for the 6400 and 6600, and looking at the price points, I'd have to go with the Flex.
You can do some very cool stuff with the Apache Labs radios that the Flex radios can't do, though:
- Coherent beamforming on the dual ADC radios (they call it diversity, but that's selling it short). What Flex calls "diversity" is not coherent, i.e. it's just amplitude diversity.
- True independent sideband AM transmission and reception.
- Built-in test modes (sweep and noise generators, etc.)
- Unrestricted transmit frequency coverage.
- The ability to modify the software to your heart's content, since it is all open source.
- Adaptive pre-distortion linearization ("PureSignal"). This is the real discriminator. No radio is cleaner or sounds better than a properly set up Apache Labs radio and amplifier.
When and if Flex ever implements predistortion linearization, then at that price point it'll be the final nail in the Apache Labs coffin.
I do hate the SmartSDR user interface, though