Something can be said for either, both are ways to keep content alive to some extent and one should not cancel the other option, in my opinion.

Personally I am a fan of the Tomestone farming, I still consider myself a pretty casual player who's more into gathering, crafting and mainly doing the story content.
With Tomestones I'm assured to at least stay around or above the content level while being able to mainly do the things I enjoy doing the most in this game.
The random drop system is perfect for those that enjoy and have enough time to run that content.