Especially X Mainly: Qual a diferença?

I'd like to know about the difference between "especially" and "mainly", if there's any.

I don't know if it's right but it seems to me that mainly is related to quantity or the most part of something.

Can I use both for the same purpose?

For instance:
I'm keen on chocolate, especially the dark one. OR
I'm keen on chocolate, mainly the dark one.

I think the 1st option sounds better but I'm not sure about that.

Thanks in advance, folks!