I use audible but I really couldn't say about comparing it to iTunes since I've never got audio books from there. One thing I do like is

That the monthly account fee of ~ $15 gets you 1 credit for an audiobook. It's cheaper than paying for the book outright since most books

cost 1 credit for members.

How much does it cost to buy more credits?

I don't really know, sorry. They do have sales sometimes where you can buy more credits but I'm not sure what the norm is.