If you book an ADR specifically for Candlelight Processional than you are "guaranteed" a seat......you still want to get in line for the "best" seats as they do let you pick your own seat. You are given a slip of paper to show to get in. I can't remember if it was 10 minutes before or maybe 15....but just like Fantasmic, if you don't show up they will then open it up to those without ADRs who have been waiting in line.
So...if you want to be sure that you have a seat.....get the ADR....if you are willing to stand in line a bit longer in the hopes that you'll get a seat, don't. You can also hear it if you just hang around outside....seeing is a little more difficult as the CMs keep making you move around to keep the path clear for those trying to get through. But, we have sat near the building on the ground and just listened a couple times......not quite the same experience, but it was peaceful and relaxing....and since we'd see it a few nights before we didn't feel it was a lesser experience, just different.
I will say that different narrators definitely make or break the experience....I think that's why Neil Patrick Harris is so popular....his voice and style are quite appropriate to the story being told. Have to say that Whoopi Goldberg, who I adore in Ghost, STNG, etc, was definitely not my favorite to listen to (thankfully she was one of the sit-outside-and-just-listen). I wouldn't say I hated it....just that her voice was a bit jarring and didn't seem to fill the peace and tranquility I was looking for in the story.
One that I soooo wanted to hear was Gary Sinise....I'd listen to him read the phone book. Alas, he has never been there during our time.