I've been to both the last 2 years. Utila last March, Roatan mid-May 08'.
Right now on Utila it's Whale Shark season if that's of interest but it's snorkel only. I think I'd split my time about 60/40, there's more to do on Roatan - Shark & Dolphin dives etc, and more nightlife/other activities - ziplines, beaches, lots more bars/restaurants etc. But there are some good dives on Utila also. Almost all the dives there are boat dives. There is some shorediving off a couple of the AI's but they're quiet at night.
Diving is split north/south on Roatan, both have their advantages - with a little planning you can do both. We dove mostly off the West End but also did the Shark dive, Dolphins at AKR and Mary's Place and the Prince Albert off Cocoview two different mornings.
On Utila you want to stay in town, three of the AI's are somewhat isolated (Deep Blue, Laguna Beach, Utopia Village) and only take you into town certain days by boat. We stayed at Deep Blue so IDK about the resorts in town. DB was quiet at night - it's by itself with 10 rooms so likely not ever crowded. Good people own/run it.
On Roatan nightlife
is the West End. Coxen Hole is mostly locals/cruise ship tourists and pretty seedy in some areas at night. Not sure if there's even a hotel there. The diving is elsewhere so no point staying in that area anyway.
You could stay in West Bay at one of the resorts but it's 2+ miles to the West End. As expected the Friday night pub crawl is a fun mix of tourists, diveshop employees, ex-pats and locals. The bigger AI's on Roatan are either on the south side or Anthony's Key - all are a cab ride to the West End. From Cocoview or FIBR, you'd only want to do that once - it's a 1/2 hr. ride or more.
There are good options to stay in or walking distance from the West End in all price ranges. Some idea of what to expect:
Bay Islands Diver Home
How are you getting back and forth? Besides flying two options are:
the ferries (you take both):
Safeway Maritime Ferry - Roatan to La Ceiba
Utila Princess Ferry | Utila Guide Honduras
or Vern's catamaran:
Ferry between Roatan & Utila daily with Capt Vern on the Nina Elisabeth II Catamaran
