Spark Streaming went alpha with Spark 0.7.0. It’s based on the idea of discretized streams or DStreams. Each DStream is represented as a sequence of RDDs, so it’s easy to use if you’re coming from low-level RDD-backed batch workloads. DStreams underwent a lot of improvements over that period of time, but there were still various challenges, primarily because it’s a very low-level API.
As a solution to those challenges, Spark Structured Streaming was introduced in Spark 2.0 (and became stable in 2.2) as an extension built on top of Spark SQL. Because of that, it takes advantage of Spark SQL code and memory optimizations. Structured Streaming also gives very powerful abstractions like Dataset/DataFrame APIs as well as SQL. No more dealing with RDD directly!
For me, it’s DataFrames all day. But Yaroslav has a more nuanced answer which is worth reading. There are also a couple of good examples.