My recent experiences with both.
On Roatan's West End, the bigger issue for us was the mosquitos. As long as I Deet'd myself nightly, it was fine, once I forgot and my legs suffered for it - they were under the tablecloth in an outdoor restaurant.
I spent some time on Half Moon Bay beach - not necessarily one that has a Sand Flea problem as I'd heard they're more in West Bay - and didn't get a single bite. But mostly walking not sitting. We also walked on the beach barefoot at "Barefoot Cay" one morning with no problem. We were there in May 2008.
On Utila last March I was pretty torn up by both fleas and flies. We stayed at Deep Blue. One day we went to do their shore dive and I forgot and put my booties on with sand in them. Huge mistake - that night I had so many bites on my feet it looked like a rash. But they disappeared a day or two later.
And I unfortunately later removed my boots on the deck then laid on the bed - that night I was bit up all over again - I sprayed Off all over the sheets to stop that. Except for that one night we never had a problem in the room - near the beginning of the trip we were even leaving the patio door open so it's likely we brought them in with us that day.
A couple days later I was waiting by their boat around 4PM when I realized I hadn't applied Deet. Before I could leave I must've been bit 10-20 times by flies - they were in my gear and everywhere. I doused everything with Off but once we moved offshore they were gone. For reference, the DB dive dock is in still water off a shallow Mangrove lagoon so it's prime breeding ground - you see them swarming as the sun sets.
One of my fellow divers went for a walk one of the first nights we were there. By the end of the week she looked like a bad case of chicken pox - we were kidding her that they'd never let her back in the U.S.
Otoh, there were at least 3-4 days when I know I forgot to apply Off after diving in the morning and I sat out on the deck without incident. But the deck faced the ocean and always had a good breeze. It seemed like the worst time was around sunset. I was there last March/April. Deep Woods Off seemed to work pretty well.
The owners otoh would sit out every afternoon and most evenings and were always bite-free.
Drink gin, it's a natural repellent...