Joined: Jan. 2006
For kegs I'm going to go with vanillin. Anything in wood is going to have more vanillin and similar wood products in it, and thus be a much more "rounded" flavour.
If we ignore contributions to flavour from the "packaging" beer in presurised kegs (metal)/unpressurised casks (wood) will have advantages over bottled beer in (at least) three ways:
1) It is in the dark and therefore unlikely to be lightstruck. If memory serves the Hen bottles are clear glass and thus offer little protection from (UV) light. Also IIRC Hen is a "bitter" bitter, thus has lots of lovely isohumulones to turn into skunky thiols.
2) Aeration and oxygen. The method by which these things are poured aerates them, perhaps allowing for some oxidation of various compounds (not checked the GC profiles of various beers, so cannot comment accurately) and certainly for the formation of bubbles. Everyone knows bubbles make thing taste good! Taste a beer poured through a sparkler as opposed to a flat, open nozzle. It makes a difference.
3) Freshness. The turnaround of keg beer is usually pretty damn fast. Bottles can linger longer.
I have no idea about different recipes for bottles vs kegs, but the above are sufficient to produce big changes in taste.