A lot.
A traditional Saison is light bodied, somewhat hoppy, and very dry. However the majority of the flavor in a saison comes from yeast. So you will have a spicy and fruity flavor. Some saisons are soured.
Biere de garde flavor and profile is very malty. There should be an impression of sweetness upfront followed by a long dry finish. More often than not there is some toastiness. In a biere de garde you also will not have much of a hop profile.
Also biere de gardes are lagered.