Two destinations, two operators, and one pricing rule that makes the size of your group almost irrelevant. Take them in that order.
Orange Bay or dolphin spotting?
Orange Bay is an island stop: the boat crosses, you land, the island ticket is what you are paying for on top of the boat, and the day is about the beach and the shallow reef around it. Dolphin spotting is a search: Bullet gives 45 to 90 minutes of looking plus two snorkelling stops, Serenity runs four hours to the Dolphin House. Nobody promises you dolphins, and any operator who does should worry you.
Which operator handles more for you?
Serenity is the bundled option: hotel pickup and drop-off, soft drinks, snorkel gear, a guide and photos are all inside €150 or €160 for a boat of up to eight, and the guide works in English or German.
Bullet is the bare-boat option: €100 or €160 for the boat, with snorkel gear, life jackets, skipper and fuel included, and everything else — the drive to the marina, snacks, lunch — on you. Up to nine fit on the largest boat in the fleet, so ask how many seats this particular trip takes when you book. Bullet's guides work in English, Russian, German and Arabic.
Does the size of the group change the price?
No: the price does not change with the count. Bullet's half-day to Orange Bay is the cheapest of the four at €100 for up to five hours, and it is the same €100 whether two of you go or the boat is full. A couple who wants a private boat should read the per-boat price the way a group of eight does — as the whole cost of the day.
Important
Serenity's 5.0 Google rating rests on 13 reviews, a small sample by any standard, while Bullet's 5.0 comes from 217. Both are real; only one of them is settled.