When you say "directional" I can read you are not talking about anything regarding waves
So assuming you are talking about a freerace board then, for early planing and some bump and jump fun
Two fins will give an extremely lively and "light" board - but a bit too lively for performance and comfort sometimes.
That is the typical tradeoff IMO.
You can do "light" freestyle and jumping way better with two fins, but the board will very likely bounce more when driven fast, as it only has a "one point" C.O.E seen from the wind (and learned when riding these)
Just some of my experiences with different boards and different fin setups...