In short:
Bourbon = American oak
Cognac = French oak
However, these apply to only wines that have an obvious Bourbon or Cognac character from rather heavily toasted barrels. Those are easy cases, but most wines unfortunately (well, fortunately) arenât that obvious.
Both types of oak can impart a vanilla character, but often American oak lends a more sweeter, pure vanillin note along with rich coconut overtones, while French oak has a drier and spicier vanilla quality to it.
American oak that hasnât been dried long enough can impart a green, somewhat dill-like herbal note to it - but this is, in turn, different from the Rioja dill. Normally a young Rioja wine doesnât have that obvious dill character, but a Rioja wine that has been aged long enough in American oak can have a striking, less green but very obvious dill note to its nose. Aged wines of La Rioja Alta are very susceptible to developing dill notes over the years.
Less toasted French oak tends to have spicier and woodier notes that are less sweet than those of American oak. If aged long enough, these woody notes can get rather bitter and aggressive.
Butter isnât a characteristic of any oak, but instead diacetyl that can develop over MLF or even fermentation with certain yeast strains. Since MLF is usually done for white wines aged in oak, people often assume buttery qualities come from oak.
Creaminess is more difficult thing. Oak aging tends to give some creamy qualities, but so do MLF and aging on the lees. Iâve had wines that Iâve described as âcreamyâ that have been fermented in oak but havenât gone through MLF or aged for particularly long on the lees - and wines that havenât seen any oak or gone through MLF but aged for very long times on the lees. What Iâve learned from âcreamyâ qualities is that they are deceptive and itâs hard to make anything more than educated guesses out of them.
Finally, a point already touched by Adam: historically French oak was considered the lowest in quality in Europe, due to its rather open-knit texture and rather powerful - even overwhelming - characteristics. Oak from Slavonia, Hungary, Baltics and even Germany was favored over French oak because barrels made with wood from these parts made barrels that left noticeably smaller imprint on the wine. Only when the French managed to turn their huge oak forests into a âthingâ, people began to associate obvious oak tones with desirable qualities and suddenly French oak was a Big Thing.