The Q950T is 9.1.4 and the Q850A is 5.1.2 (but you can upgrade the rears to the SWA-9500S for additional money to make it 5.1.4 and actually wireless). The Q950T has an additional 4 channels in the soundbar itself, and versus the included rear speakers an additional 2 ATMOS channels. The speakers that come with the Q950T are truly wireless beyond a power cord, and the speakers that come with the Q850T are only wireless between the soundbar and the rear amplifier - which then requires speaker wire between the 2 satellites and the amplifier. Also, the speakers that come with the Q950T are significantly more powerful.
The Q850A however, comes with a few additional software features; such as; Airplay support and Room calibration (but you also have to have a matching 2021 Samsung TV for this).
Note - Both devices support LCPM as of the April 6th 2021 Firmware update.