Every recipe I find for White Chocolate Truffle filling seems awfully ganache-y to me. Is there a difference?

Depends what the recipe is... Many high quality truffles are made with ganache, it can easily be a good cake filling too.
xx

They are basically the same recipes using the same ingredients, where ganache has different ratios of chocolate to cream depending on the consistency you want and the type of chocolate. Once it cools and hardens, it is scoopable to be use as truffles.

If you look up recipes for truffles, you'll see that it's ganache that's allowed to harden to a semi-soft state, so it can be rolled.

So what are you using for White Chocolate Truffle cake filling? I'm not sure if I should do a ganache-y type of recipe or take a box of white chocolate pudding and heavy whipping cream, or go buy a vat of Bettercreme at Sam's and mix some concoction that way...

If you want a truffle type filling, then go with the ganache. If you want something lighter and more mousse/pudding like, then you could mix pudding with the Bettercreme as you suggested.

Hope this helps!