Sharks on Roatan are usually only found here:
Roatan Shark Dive Any dive operator can book you there.
We only saw a couple reef sharks in Utila also - on the deeper north side. Not counting the 6 Whale Sharks we snorkeled with. But the optimum time for that is early spring. Utila in town is pretty affordable, lots of backpackers so prices at many places are reasonable - there's also some dorm-style accommodations. The AI's are farther out of town, isolated on the other side of a lagoon channel so all access is via their boats. They tend to be quieter.
Check flights before you commit to something also - Utila will be more difficult to get to, either multiple flights (no lights at the airport so all travel has to complete by mid-afternoon) or via the ferries. Depending on where you're from, that may mean an overnight in San Pedro Sula or La Ceiba along the way.
There used to be a direct Catamaran from Roatan to Utila but he was recently murdered. Via the ferries takes about the same time (4hrs.) it's just not as convenient because you transit thru the mainland.
Coconut Tree does 3-4 dives/day - at least 3 and 4 if there's demand. Also a couple night dives per
week. The other shops (there's a lot of them) also night dive on different nights so you could probably find one just about any night - except maybe Friday when most people party on the West End. Dives with CTD are 10/$350.
Coconut Tree also has affordable cabins - $45/nt IIRC. My buddy rented a studio, queen bed, small frig, table, not much else. Kind of older, rustic but clean and decent.. Hot water worked there also - that's not the case everywhere.
There's a small store in the CTD building for essentials and a slightly bigger one - Woody's - a short block away. Most of the affordable food options are within a block or two in either direction. Sundowner's Bar which is a local dive bar is across the street. They have decent sandwiches at reasonable prices. There's a lot of under $20/meal options in the West End also.
There's also a lot of other affordable accommodations in the West End, ranging from a Hostel (Chillies) to private condos and small houses. Depends on your budget. CTD lists some options on their site:
http://www.coconuttreedivers.com/roatan/accomodations.html
I personally think Utila has a slight edge for diving over Roatan. Most of it is longer boat rides though since town is on the south side and IMO the best dives were all N/NW on the north side.
Off the West End on Roatan, they come back to the dock often between dives, nothing is more than 20mins. out or less. We actually started prepping our gear while the boat was still tied up to the dock some days.
In all fairness, I like bigger animals. For macro Roatan's south side is probably the best.