I've smoked both, and don't mind either. I'm working through a pouch of Captain Black at the moment. I would choose sail though. Its a mild very flavorful English blend. I've tried a lot of OTC blends, and rank it among the best.
There isn't a lot of difference among the 3 Sail mixtures - yellow, ivory & green label. The green is called aromatic by the blending house but I suspect that is because it has a hint of latakia and/or Oriental leaf. Any of the 3 mixtures make a good all-day smoke. The yellow label stuff used to be sold in tubs but now all 3 mixtures are sold only in pouches. I buy 5 or 6 pouches at a time and jar them.