Hi Brian,
December is the dry season, so the weather should be really nice for your visit. Of course, if you want to check out the rainy season prior to a move, you’ll want to come back another time from May to October.
As far as M.A. as a destination, it’s a very nice area and home to a good number of expats. It’s also a popular tourist spot. And from there, if you rent a car you can also visit places like Jaco, about 40 minutes north, and towns like Dominical and Uvita in the Southern Zone, 40 minutes south, to get an idea of this beach and jungle area of Costa Rica’s central and southern Pacific coasts.
Whether or not there is a better spot to visit is up to you and the type of place you’re looking to live in Costa Rica. The Central Valley and Arenal are inland areas, mountainous and with very mild climates. Totally different from the Pacific coast. Check out the main page of the CR Insider site for more info on these regions. If they appeal to you, you might want to adjust your trip accordingly to check them out.