It depends what kind of work you are doing.
If you are home every day or local and can wait until you get back then stick with your local garage.
If you are tramping then fuel cards are great for a number of reasons - authorisation, number of garages you can use, service station access without service station proces.
However we used to use just one fuel card - securicor and the price started off great and then went up 1p per week until we ended up paying a lot more then other people we knew with the same card. We also now use croft and get calls from both companies each week and use the cheapest or haggle!!! If it starts to creap up we have found that stopping using it suddenly triggers a phone call from them wondering why!!!
Make sure you always have enough options available - running out of fuel is not a good idea in this game - also saving a few pence per litre is worth it with the amounts we use.