the difference between 91 and 98 is about 3% in power difference and economy, they are set up to run both from factory, obviously 98 will be a bit better, this is what i was told and shown on a efi performance course i did, i personally use 98 about 99% of the time, have put 91 in to see the difference and for the extra couple of dollars the 98 is worth it, i got better economy and felt better out of it, have used 95 before and didn't notice a real difference between 95 and 98.
i use shell 98 and if just a little top up will put 91 in, have just put carltex 98 in this week.
my trade school teacher said you can use 91 during the week and will be fine and put 98 in on the weekend for a bit of fun