You don't mention which itinerary you are considering, but I've done the "Exotic Panama Canal" on Island Princess and could suggest some excursions if you are doing any of the same ports.
As for cabins, an interior will always suffice--there are plenty of good viewing areas on deck and in the Horizon Court; but if there was ever a cruise that cried out for a balcony, THIS IS IT! Any balcony is good; we chose a mini-suite on Baja Deck (B206) for the larger balcony and the proximity to Horizon Court. We pretty much camped out on the balcony all day going through the Canal, making numerous trips to the Horizon Court for snacks and drinks (room service is of course available, but we like to pick and choose).
If you don't want a mini, the catagory BA cabins on Carib Deck have balconies almost as large. But there are really no bad balcony cabins (we'll, maybe a couple aft that have somewhat obstructed views).