Aerodynamicists and efficiency experts, tire advice needed.

Firstly I am looking to make my Prius sort of like the Subaru Outback or Volvo XC90 by getting more clearance and more grip so I have less worries driving the amazing forest and desert dirt roads of Oregon. (Obviously I still only have 2wd.) I know my MPG will decrease and I'm ok with that. It'll still beat my VW Vanagons 19mpg best. I'll still keep the old wheels and tires for long on road trips.
I have lifted the Prius 1.5" already. I have just bought some used OEM wheels as they are the lightest for a good price at 14.2lbs. I now have to decide which tires to mount.

I'm currently tied between the 207/70/15 and 215/70/15 Yokohama Geolandar AT GO15.
GEOLANDAR® A/T G015 | Yokohama Tire

There's not much difference, but if one gives more mpg, that's the one I'll go with. But, how does one factor in the different variances of tire width, tire weight and vehicle height?

The 205/70/15 has a diameter of 26.3" and weighs 27.3lbs
The 215/70/15 has a diameter of 26.9" and weighs 26.7lbs

Which would you choose and why?

 

I'm pretty sure both will clear, I'm just looking for the size with the least bad MPG effect and don't know how to weigh the variables. I'd guess the shorter, narrower tire, but weight is important. I have snow tires with steel wheels on at the moment and the extra weight can definitely be felt. The new setup should weigh about the same but the weight will be pushed further towards the circumference due to the heavier A/T tire on an alloy rim as opposed to a heavy steel wheel with OEM size winter tire.

 

Well it looks like the decision has been made for me, that makes it easier. (Even though Miata guys run much wider tires on the same width rims and drive way harder than I'll be driving the Prius.)
Now to just wait for them to go on sale, hopefully Presidents day.