iheartradio really isn't comparable to Last.fm and Pandora (or Slacker, for that matter). It just streams existing OTA radio stations. Slacker is a bit different from Last.fm and Pandora fundamentally, but all three are based on playing music they think you'll want to hear.
What it really comes down to is personal preference. They all use different algorithms to determine what you like, and I think the consensus is that Pandora's Music Genome Project is the most advanced, but that doesn't necessarily mean better. I prefer Pandora, but it does tend to play a lot of the same songs over and over. It's been a while since I've tried Last.fm so maybe I'll give it another shot.
It's a preference thing. I have used Last.FM for a few years, so it already has a lot of my play history and is better tailored to me, so I use that. It also works on my Xbox 360.
I have Pandora, Droid Live and Last.fm. While I'm driving, I never lose Pandora. I have lost Droid Live and Last.fm. Last.fm seems to zero in better on my music preferences than Droid Live and Pandora, but Pandora sounds better. Toss up. That's why I have all three.
So which apps played unedited songs? Last time I had last.fm installed, it was playing edited songs, as in NO explicit lyrics, it was kind of annoying.
Download and try them all. They are all similar, but slightly different. e.g. I downloaded iheartradio, but it did not have any of the local stations I was looking for so I downloaded and now use cherry rplayer. I only chose slacker over pandora because I already have a slacker radio and had several custom stations created already.