The two words are pretty much synonyms in all contexts, but if a distinction can ever be made, perhaps leafage sometimes alludes slightly more to the individual leaves (albeit, collectively). I don't advance this possibility particularly strongly, because I don't think I really believe it myself.
Thus you might see leafage pattern used relatively more often to refer to the "average" pattern on each leaf, as opposed to the foliage pattern referring to the overall disposition of leaves on the plant. By "relatively" there, I mean after allowing for the fact that "foliage" is much more common in the first place.
The general advice would simply be; forget about using the word "leafage". There are no contexts where "foliage" would actually be wrong, and using the uncommon word may stand out as "odd".