We've been in December, and it poured rain every day that we were there for long periods of time. We still had fun though. We did a lot of activities in the rain. I was surprised by the weather because I consider May-November rainy season, but that area isn't all that predictable. Can you post the map that you saw?

The end of November and beginning of December are a gamble weather wise but usually by the second half of December it's clearing up and dry season is settling in. Having said that two years ago the rainy season hung on until mid January

Unless you're on the northern Pacific coast in mid March (where it's guaranteed hot and dry) or the Osa in October (where it's guaranteed soaking wet) it's all a little unpredictable so I wouldn't try to make the decision based on hoped for weather. Go where you want and plan on making the best of it.

We climbed Tenorio volcano a couple of weeks ago under glorious sunshine even thought statistically and historically it should have been pouring rain

here's the link to weather:

http://www.imn.ac.cr/IMN/MainAdmin.aspx?__EVENTTARGET=LinksInfoClimatica

Just go and play it by ear. See how the weather is when you are there then plan the next steps.