I think it's certainly easiest to update CDH, because you get other updates and get a harmonized distribution. CDH 5.7 includes the latest Spark 1.6. The only downside, I suppose, is that you are updating all components and there is a non-zero problem of a problem or incompatibility, but, you'll almost surely gain much more than you risk.
In any event, you can always build and run any version of Spark, and it should Just Work in the main. It does require a bit of understanding of how to build and execute a Spark distribution.