Have you read Markowski's "Farmhouse Ales" book? It focuses on these two specific styles. They really are quite different, too - I'd wager that the ones you've tried haven't been great examples of the style. While they both are farmhouse ales, bière de garde typically focuses on the malt character, with subdued yeast and just a hint of hops. I'd suggest checking out the book... I think it's a pretty misunderstood style due to the difficulty in finding them over here, and it'll give you a good baseline, and some basic guidelines on how to brew one.