Being self-employed for 30 years, I always used my credit card to purchase fuel as it was easy to track the costs, especially for the Tax man, as would a fuel card.
With 2 vehicles in your fleet (and possibly more) the fuel card would be easier to reconcile for accounting purposes. Does a fuel card limit you to one brand of fuel outlet on can you use it at say Shell or BP depending on where you are or who has the cheapest fuel?
Is there an administration fee imposed by the fuel card supplier and, if so, does this cost offset your accounting fees when using a credit card?
Hope this helps.