I've been to Barbados as a tourist 3 times, and St Lucia once. I didn't like my choice of hotel in St Lucia - it was an AI, albeit not a very posh one. It was a long time ago, and at the time I likened it to Butlins in the sun. But that was my fault - I chose poorly. The country itself is amazing with tons of things to do; rain forest, active volcano, marine stuff, historic sites, shopping, etc..
I like Barbados as a "home from home" location. I've always found it safe, comfortable, friendly etc.. I've stayed once in a small hotel, but in independent apartments otherwise. I love the south / west coast, but the east coast has been opened up more with "wind" -based sports, but I wouldn't go there to "just" sit on the beach / swim - it's rough. I think there are fewer things to "do", as in organised day trips, but that may be because I've been independently, so I've not had any reps selling them! I have enjoyed catamaran days, snorkeling, and other "standard" beach holiday things.
For a different activity every day, I'd choose St Lucia. For a fly and flop experience I'd choose Barbados.