Here are a few thoughts in no particular order:

The Outlaw Model 976 is pre-amp only (Marantz SR5015 includes amplifier channels), so for the money more dollars are going into the pre-amp stage with the Outlaw.

If you never intend to add height channels then there is no benefit to Atmos decoding as the 7.1 channel Atmos mix is exactly the TrueHD bitstream.

Your Outlaw 770 amp is single ended which of course works with either pre-amp, but if you ever wanted to go balanced/differential then the Model 976 has those outputs. Even if only your subwoofers have XLR inputs you can run longer runs without interference out of the Model 976 (2 x subwoofer XLR outputs).

The Marantz supports eARC while the Model 976 is just plain ARC which may matter to you depending on how much streaming app/content you drive from your TV or its inputs.

The Marantz has some creature comforts and additional features (Network, Audyssey MultEQ XT, Airplay 2, 8k). The Model 976 has never received a firmware update and you can read the forum for its known irritations.

If you are interested in sound quality for dollars over having the latest features then the Outlaw may be for you.