I'm not an electronics man (electrons, wave motion, Fermi levels and exclusion principles are all a bit esoteric for greasers such as me ..) so the following only relates to my end use of the gadgets ..
As I understand it
(a) the QAR is just a DFDR without the crashworthiness capabilities ie, a bit like comparing a Goggomobile with an armoured truck ..
(b) in any case, the data can be a bit suspect/difficult in part and interpretation needs to involve sensible engineering and flight mechanics considerations. As a simple example, temperature calibration is a right pain in the neck and, unless the box is flight tested subsequently to qualify the calibration, I wouldn't put any great credence on that particular output parameter.
given the pressures of line operations, downloading from the FDR is, for all practical purposes, not possible
This probably depends on the boxes but, for example, one fleet with which I am involved is set up so that the DFDR can be downloaded quite simply by plugging in a cable externally and dumping the memory .. and I generally do this myself .. easy peasy. The protocols have been agreed with the Regulator and the Regulator is most happy that this fleet's DFDR serviceability is regularly and routinely monitored. Wireless technology would be neater, but having to cart a laptop into the hangar and plug into a socket is not really a major imposition. The particular box holds enough data so that the downloads can be made to suit minor hangar visits or during low activity periods on line.
This fleet has the FDR downloaded as the aircraft are not fitted with QAR. I did investigate the practicalities of installing QAR but the cost didn't warrant the utility. Had such an installation been pursued, the data would have been picked off in parallel with the DFDR.
the data has the same validity but is merely captured by different devices
Sounds about right to me. I, for one, could see little point in quibbling about whether the data came from one or the other. However, the problem I see is how one sets about interpreting some of the data, especially if one is interested in looking at short time histories ...