And if you DO decide to design in InDesign and attempt to use its direct-to-epub or direct-to-mobi function, make sure you have only used a very simple subset of InDesign's layout options. Otherwise you'll get VERY messy, bloated code (or a crash).
Epub is still basically about pouring text into a container - and you don't get to choose the text size, font or size of the container. Accept this, you will make happy, readable, portable ebooks. Try to fool ebook formats into doing more than their capabilities, you'll go mad.