The Qu-Pac has 22 analog inputs before any expansion (it can be expanded to 46 inputs with an AR2412, there are also smaller expanders of 8 and 16 inputs).
It records up to 18 tracks simultaneously, direct to USB thumb drive, and up to 32 tracks over USB to a computer. It also offers “virtual soundcheck” aka playback of the USB drive’s multi-track recording , back through the EQ/Dynamics/Gate/Ducker & bus system.
For more differences in specs I recommend looking at the manuals or even just the promotional documentation for each product. I believe both have SOS reviews as well.
Also, consider that there was about 6 years between the release of the R16 and the Qu-Pac.