Always a difficult question because it largely depends upon the charging terms and conditions of your card and your bank.
Without any question of doubt, it is 'easier' to use plastic in this day and age. Nobody ever questions cards - but not everybody will take travellers cheques and cashing them - without suffering a substantial loss in exchange rate - invariably means losing valuable holiday time queuing at a bank and going through a bunch of hassle.
However, a huge amount depends upon your card balance and how much they charge for foreign cash withdrawals. I opened an account with the Halifax because they still (I believe?) offer the best deal option when using abroad. However, whatever card you use, it is vital to ensure that you have zero balance and clear it by the due payment date - and it is also vital to notify the card company of the dates you will be in Tobago, to avoid any possibility of the card being stopped because their systems suspect it may have been used fraudulently.
Remember, the hotel will invariably be taking a cut, so you can generally negotiate a better deal by going direct. However, it depends who the tour operator/car rental firm is. The reputable services will give you good service regardless of who you book through. However, sometimes having the influence of the hotel/resort behind you can be a big help when dealing with smaller operators.