I think you get the most, generally, for a tourney. I would enter a unlimited tourney and buy the xp boost. Then, if I could, I'd time it out so I would start a round beore the last drop ran out then it lasts the whole round.
I think the 18 hole rounds gave more xp than, say, two 9 hole games.
Par 3 CC tourney games were fast and I've played many of those w/ xp boost.