Interesting matchup, since they have little in common.
Personally I was never a huge Grievous fan. Honestly he seemed like an unnecessary villain back in the day. Though I’ll grant that his story gets more interesting with a deep dive into his background. Plus there’s always 2003 CW Grievous to consider, and props to that just for the animation alone.
I choose Kylo of the two here, but to say I’m a fan of him would be a stretch as well. I view him more as a fascinating train wreck.