Onyx Ring Discussion

I have a dilemma,

I can replace my Explorer Crit ring (10% Crit rate + 10% Min Crit damage) for the 10% boss Sylph Ring. What I want to know is it worth it to replace it or is it literally the same when I'm bossing?

That really depends on how much of those stats you already have.
You can try to figure out which is best using hadriel's calculations, or you can just go by the rule of thumb that it's generally better to add what you have the least of.