It depends how you relax. Barbados has lots of snorkeling, retaurants and bars, beachside rum shacks. Smart hotels with pools etc.
The South of the island has more watersports and more 'party' stuff.
Tobago is less smart & quieter but has some stunning golf courses and the most amazing national park with rainforest. Secluded beaches like Englishman's cove. Hire a jeep and explore. Develop a taste for curried crab. 😊
Plenty of islands have adult only resorts.
Which do you prefer?