I got the same offer letter and if we hadn't already planned out our August trip to WDW with our 2 kids and 3 nephews, I'd definitely sign up for the Costa Rica trip.
We haven't been to Costa Rica with our kids (yet), but will absolutely take them there. Yeah - it is the rainy season and you'll get some pretty incredible downpours. You'll need the right clothes, shoes and attitude!! However, it will be impossibly green and lush and beautiful. We've gone during the "drier" season (early April) and it's incredibly hot and humid. We're absolutely looking forward to going there in the summer with our kids, but will have to wait until '08.
There are some great forums out there with good info on Costa Rica. Check out trip advisor (if you haven't done so already) and you'll find a good deal of info about the rainy season and what it's like, how to prepare, etc.