Good choice on Panama over Arizona this time of year. I use to be a biologist in SE Arizona and found either mid may or mid august to be the best times to bird down there in terms of diversity and numbers. If I had to choose between May or August, I would take august. In mid august many mexican vagrants are showing up, the local specialties (red-faced warbler, etc.) have not left and western north american species from further north are starting to return south. And it is cooler weather in august than mid may.
As for Panama field guides. the Costa Rica one can do. However there are several species in Panama that do not make it to Costa Rica. And this may justify the cost of a new heavy guide to haul around.