I own two Ecoboost engines and have never purchased even a single drop of anything other than 87 octane and have to say that they run great and pull well. The only issue I have is smoking the tires when trying to keep up with traffic while pulling near the rated 11,000lb capacity. Since I own a motorhome my pickups get used for other things. I get about 12mpg towing a three horse trailer and about 9 pulling my heavy equipment trailer.
On the subject of axle ratios: The 3.73 is only with the max payload package and that is available only with the long bed. It is the only axle available with the max payload package. I really doubt that you would notice the difference between a 3.55 and 3.73 rear end anyway. I sometimes wish that I had the max payload package but for some reason it cannot be combined with most options and I wanted some of those options. It was a tough decision but those options are really nice on the road. I am surprised at how much I like the auto wipers and headlights (with hi/low beam) on my truck. The adaptive cruise control is nice too.
Be sure to get the 36 gallon tank but the engine way outpulls the V10 that I used to have and gives about 20% better fuel economy while doing it. I would be a lot more interested in an F250 if it offered the Ecoboost.