iHeart is owned by Clear Channel Broadcasting. Clear Channel is the company that bought up a lot of radio stations over the last 15 years or so. It is a way for them to continue to reach consumers who no longer listen to traditional radio in their cars and homes. If it's not one of their stations or affiliates, you won't hear it on iHeart Radio. Good news is, they own a ton of stations and iHeart was how they centralized the considerable content their stations have. I live in a big city and let's say there are 20 major stations of various types of music and sports radio, Clear Channel would own about 15 of them.
There is no overlap between iHeart and TuneIn. If it's a Clear Channel station or affiliate, you won't hear it on TuneIn and vice versa.
I personally think music and radio was better before Clear Channel came along but I'm happy to be able to listen to a few of my favorite local radio stations easily when I want but I'm definitely looking forward to the new TuneIn and Pandora apps. There are several W8 apps that aren't on WP8, I hope it won't be that long of a wait for them to migrate over to WP.