I'm assuming you're likely in Ontario, if so, both the shell 91 and Peto 94 are using Ethanol free fuel.
I've used the 94 in my rig a few times, and find the mileage to be worse. Using the 94 with the 93/94 octane tune in my jeep yeilded .1 of a second better 0-60, but that also could have been fluke, If I were stll in ontario, I'd be ding more testing. I'll be back out that way in April again however and will test and tune a bit and see if I can find more differences
But overall, I find the shell 91 to be one of the most consistent fuels in Ontario.
I ran 2 heavily modified turbos before I got the jeep, and didn't find much of a difference when the cars were stock, but when they were tuned, that's when it really took advantage of the extra octane.
In my opinion, if the truck says 91, there's not much point in putting higher in, because it's tuned for 91. Higher octane will just make it run richer. And you'll be burning fuel unnecessarily. (40 kms per tank or more I found depending on the vehicle) But if you tune to take advantage of it, use the 94 and never look back.
If you're in the states around Ontario, you pretty much have to use 93 octane no matter what. And it runs similar to the Shell 91 in Canada in my opinion.
When you get to the mid west, in some areas, the highest you can find is 89, but you're high elevation, so it's similar circumstances as higher octane, it's very unique.
Over in BC they have Chevron 94, and I haven't tryed it in the Jeep yet to see how it performs. But from one of my friends experiences who moved here from ontario, Esso 91 is the better choice in BC over Shell. So I'm going to test his theory.