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For fuel economy, you want the tallest gears that you can run at your target speed with the torque converter locked up. If you're going to be accelerating and decelerating all the time (e.g., city driving), the gearing probably doesn't make a whole lot of difference--your FE isn't going to be very good anyway.
If you've got lots of highway miles to travel, you want the tallest gearing you can find, subject to where the TC locks up. "Gearing" takes into account the individual gear (4th gear, the tallest one you have), the final drive ratio, and the tires. Larger tires effectively make the final drive even taller, reducing the RPM to run a given speed.
Speeds in 4th gear at 6000 RPM for your first transmission with 25" diameter tires are: 46/83/129/188 MPH
Speeds in 4th gear at 6000 RPM for your second transmission with the same tires are: 37/68/105/153 MPH
Since the speed at 6k RPM in 4th is almost 20% higher for the first trans, you can figure that the RPM at any given speed in 4th is about 20% lower with the first trans than the second.
So given the two of these, you want the first trans--the 245 one.
-soD
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