6/8 and 12/8 are similar and a lot of the time are interchangeable like you said. I tend to think of slower songs as being in 6/8 and faster songs in 12/8.
For example I would count this song in 6/8
[ame="http://www.youtube.com/watch?v=175WOIYLm-s"]Rival Sons "Soul" - YouTube[/ame]
And this one in 12/8....which in this case can also be counted as 4/4 with triplets
[ame="http://www.youtube.com/watch?v=XNWMVeLtgRw"]Alter Bridge- Ties That Bind - YouTube[/ame]
I'm not saying I'm right or that the songs are "actually" in these time signatures because they are somewhat interchangeable...that's just my personal rule of thumb.
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