You can use both, but 98 is better since it has less ethanol. Our 95 is the same as 91 in USA. People in USA even use 87 with no issues, which we don't even have in Europe.
I just found out that almost all our E5 gas stations have 0% ethanol.
The only reason why they are called E5, is that they are allowed to use a maximum of 5% ethanol in their gas, but they don't, so it's technically E0.
Finally, I had time to make this tutorial on how to change side mirrors with turn signals.
Best mod for all Europeans who order a North American Mustang without turn signals in the side mirrors.