The thing I really like about the rune priest is while you choose a build option, it doesn't affect your powers - power X isn't better if you're a type 1 priest. Instead, every power has two associated options - one based on defence, and one on offence. So, each time you use a power, you choose which of those options to invoke. And there's no reason why you can't switch choices each and every time you use the power, which is awesomesauce in my book.
I'm not a huge fan of the class thematically, mind you, but hey, that's what re-fluffing is all about, right?