Hi Nettie,
I've shared my Coiba trip report in your other thread. The report from my Little Corn trip is
here.
Having been to both, I can tell you that you'll have a wonderful vacation whichever you pick. Both are amazing and unique places and both are well worth taking the effort to see.
I haven't been to Utila, but I have been to Roatan. I know it's not apples to apples, but it should be at least lemons to limes.
Of your two options, Coiba is the most distinct from Utila. It's a national park rather than a community, and it is Central American Pacific diving and topography vs Central American Caribbean. The water will either be cooler or colder depending on the time of the year. It's less coral reef and more rocky and the marine life is more pelagic vs reef fish. You stay at a ranger station which is your home base, where you eat all of your meals and sleep in dorm-style rooms. I loved that once we left the ranger station, it was rare to even see another boat while we were on the water. Surface intervals are spent on the beaches of small, wild islands. In the evenings, you enjoy dinner at the ranger station listening to the chorus of frogs and other animals in the jungle. You can't stray far from the ranger station, but I never found myself wanting to, nor did I ever feel bored.
However, Little Corn Island is the spot which is going to change the fastest. The Little Corn of today will be a faded memory in 15-20 years. The diving is very typical Caribbean diving. The dive sites are a short boat ride from the beach and it's typically fairly shallow diving. What's special to me is the time capsule aspect of the island. This is what the Caribbean was like 60 years ago. There are no motor vehicles on the island whatsoever. You can walk around the whole place in about 45 minutes. Cargo is moved from the docks via handcart. Fuel drums for the island's generator are pushed off the side of the boat in chest-deep water and then rolled up the beach. The generator is turned off during daylight hours, power comes back on in the evenings. The whole island has a quaint backpackery vibe to it. There are a few small restaurants, all of them delicious.
Little Corn has a lot of pressures that change it quickly in the coming years though. The Chinese are building a canal through Nicaragua that will exit near Bluefields which will put the Corn Islands in cargo shipping lanes. There were also rumors of trying to build a small airstrip on the island.
Coiba will change as well, but definitely not a quickly or as drastically. Since it is a national park, the government has some interest in limiting the number of people who can visit and keeping it wild.
Outside of the specific destinations, there's more to see & do in Panama vs Nicaragua IMO. If you do go to Little Corn, spend as little time as possible in Managua. If you have free time on the mainland, go see Granada instead.
In Panama, I loved the laid back vibe of Santa Catalina and could wander around Panama City for days and never get bored.
Hope that info helps a bit. You should love whichever place you pick. If you have any specific questions about either, just let me know.
