I'm watching closely Apache Flink recently and it seems to me that it's more universal that Apache Spark, and with less overhead. Today if somebody asked me for general purpose stream processor engine I would recommend Flink. Of course Spark has many advantages too, but unless you don't need explicitly need Spark's RDD, then go for Flink.

What I'm still missing is better support for machine learning libraries, but I suppose it's a matter of time.