I have flown into and out of both. Liberia is a great airport to fly to/from. On my most recent trip, I was in and out of the airport in thirty minutes. Rental car was a breeze and on to the beaches very quickly. If you are travelling to that region, then I would recommend it. From what I have read, the rainy season is in full swing and the Carribbean side is the most pleasant during the months of September and October. For that reason, we are flying into San Jose.
I don't really like driving in San Jose, and would avoid if possible, but the San Jose airport is much more centrally located than Liberia.
With all of that said, enjoy your travels.