I not sure which area your trip is to,looks like Los Suenos. But down in Golfito, both months are very good. We have excellent all year fishing unlike most of Costa rica. If I had to choose I like after the full moon in Dec.. Some good shots at sails, possibly up to 20 shots a day and the smaller marlin are pretty common. Last year which was slow we saw a marlin almost everyday. They are in the 150lbs.-350lbs. range. There is also Dorado and smaller tuna around like 30 lbs..