I just played a gig on my friend's DJM-900 and CDJ-2000s, all plugged in via USB to my laptop running Traktor (with an X1 and custom mapped controller for FX on the side). And what a great way to play that is. But a pair of 2000s is super out of my price range, so let's chuck that out the window.
But down to the 850 and the 900: their main differences seem to pertain mostly to DJs playing without a laptop; things like pro DJ link (or whatever), looping, slip mode, sound quality, and so on. For a DJ playing on Traktor and using the CDJs with HID, none of that matters. If I'm not mistaken, with Traktor 2.6 both the 900 and the 850 have Traktor HID support, so my question is: for that setup, what's the difference? Are there any discernible advantages that the 900 has over the 850 in HID link mode, and if so, what?