Someone did the math and according to him single is better if you want 5* ce while 10x is better for 4* ce. For servant the rate is the same for both. Keep in mind that since no one know how the guaranteed 4* or above is generated in a 10x roll, the result is based on the assumption that the formula is true.
I prefer single roll myself, since I can stop when I get what I want where as when you multi roll, you have to do 10x roll even when you hit the jack pot and have that rate up on the 1st or 2nd card.