DJI's quest to make a drone under the 250 gram limit required some compromise. MM has no obstacle avoidance and the
battery is Lithium Ion vs Spark's Lithium Polymer. Nothing wrong with LI
battery, in fact they're usually less expensive. Unlikely DJI
battery allows replacement of cells, but maybe a 3rd party will. I don't recall reading the MM
battery is "intelligent" as is Spark's.
On the plus side MM has reportedly longer flying time, foldable design and a higher resolution 3 axis camera. MM operates in the 5.8GHz spectrum whereas Spark is dual band, likely another cost control decision. It's unclear what difference in RCs exist, but you can bet DJI had rather use an RC unique to the MM as an upgrade path (aka, higher price point) for their Fly More Combo.
I'm hoping the official announcement tomorrow will fill-in some of the blanks. I'm not thinking of buying another drone anytime soon, so I'll see how the MM shakes out under consumer testing over time.