@sethforprivacy does a nice job explaining the trust tradeoffs between Ark and Spark.
Tldr: Ark has stronger finality guarantees and requires less trust...as long as the user is reliably online (not constantly, but frequently available to sign. Spark has lower online-ness requirements, but leaves the user trusting the Spark Operator to a greater extent.
Read the whole article for a pretty good basic explanation of the two protocols.

