There are strong realism arguments for both perspectives, but I think that going for the amorphous XP pool gives more freedom.
However, what should be done in recognition of the need for the non-combat part of gameplay to be competitive instead of the "slay a dragon" mentality would be to ensure that activities that provide profit also provide experience. Sort of a Euro Truck Simulator 2 kind of thing -- experience by distance and danger of cargo for trading, experience by difficulty and danger in extraction for mining, experience per "find" while exploring/charting/surveying frontier worlds, experience over time while administrating a trade centre, etc.