On Utila, even at an AI, you will be immersed in the strange and unique atmosphere... or whatever it is that Utila is.
Utila is one of the islands that I would put on the "see it now" list, before time and success screws it up. Most all of the resorts center around Utila town, which can rightly be described as dusty concrete in nature.
I am not as enamored with the diving there, and as you casually reference the Whalesharks, that is probably a good attitude to take. If you see them at certain times, good for you. If you go there expecting to have that ticket punched on your dive log book, I wish you well. The overall diving is as good as you'll get anywhere in the Caribbean nowadays, but certain areas of Roatan blow it away.
To compare an entire island town to one AI resort is a bit odd, though.
FIBR sits along the Southern shore of Roatan, an area that is unique in the Caribbean. It has vertical walls that break at reefs very close to the surface. They are exposed to the SE, so they get Sunlight all day long. A peculiar niche environment.
You might also be talking about price, when you use the phrase "haven't been able to find much difference".
FIBR is about the least expensive thing going on Roatan if your main focus is diving. If you want 4 a day on Roatan, you can get this at a great price at FIBR with a little bit of effort.
The dive schedule (overall) is a little more slow-paced on Utila, based a bit more on the European model. You're going to have to select with care to get larger quantities of diving.
In either (and any) case, just remember that when a dive operation offers 3 boats in a day, you might have to hustle a bit, like at FIBR, to make them all, or in some other cases... you really can not do all three because of timing. Know what you want, analyze the schedule, and pick from there.
FIBR and it's like will be around for a long while, although the unique Southern shore is facing it's ecological challenges. Utila, though, is changing fast. I have never been a great fan of it's diving, preferring the South shore of Roatan, but if you want to see a throwback to the 1970's (If you take-out the electronic toys like cellphones and TV dishes), go to Utila now before it is ruined by its own success.