I have used both on in the three 2.7 motors I have had in the new broncos. There may be a slight hp seat of the pants increase with the 91, but not enough to justify it all time. It typically go back and forth, one tank 87 the next 91. I dont let my tanks get real low so it kind of averages out.
For what ever reason I seem to be getting slightly better MPG with my 24 badlands SAS then the two other broncos I had with the 2.7. Right around 17-18 mpg if I keep it around 65-70 on the highway I can squeak out 19 on longer trips.