Roatan is a little more expensive, but not much, 10-20% on food, drinks. Diving and accommodation is same price on both islands.
Roatan's West End is much more of a beach with houses, hotels, bars and restaurants along it. As you know, Utila town is more focused on a main street as opposed to a beach.
Roatan is much bigger and with the cruise ships visiting has many more activities than just diving available. However the cruise ships bring in a certain amount of daytime crowding. There are about four cruise ship days a week between Nov and April, 1-2/week the rest of the year. You will notice when a cruise ship is in for sure - the the scooter wrecks on the side of the road give it away.
Diving is similar, many argue that Roatan has better fish life, i just don't dive on Utila enough to confirm that though.
HTH
Will