It’s easy to go wrong with the Grunfeld if you don’t understand the strategies, how to use your resources and staying on tempo. With that said, I prefer open and tactical games, so I vote for the Grunfeld.
Both are dynamic defences. The Benoni does have transpositions if that is of value to you.
For a bit of context, I play the Najdorf as Black, and the Ruy Lopez as White. What would better fit my openings as of now? I've tried many other openings against d4 but they didn't seem to fit. Both of these are the only ones that I seem to understand the common ideas in.