Greetings Martin, I visited the island two years ago and spent three days traveling around exploring. It’s a fantastic island so if you did a day trip you would only be scratching the surface. Best regards Diver
As far as I’m aware you can only get to Santo Antao by catamaran or ferry from Sao Vicente and not directly from Sal. So although I think it’s definitely worth a visit, you need to spend at least 1 night in SV & 1 night in SA to justify the cost of the flight to SV. If there’s a few of you then obviously this can add quite a chunk to the cost of your holiday. If you’ve got the time and the money then go for it & see SV & SA – I highly recommend them both.