I made an experiment as I can choose between x and exec.
This week just x compared with exec last week, there is no difference when it comes to money, maybe i spent 20% more on fuel but i spent less time at work as with X i get requests non stop.
Comparing depreciation of the exe car with the fact that you can make same amount of money driving for x in a £4000 car, its a no brainier. I made around £20/hour driving for X, same as EXEC but no dead time between jobs