I would definitely go with 1 of the Diablo tuners. Adding the Hemifever custom tuning only sweetens the deal even more. Diablo tuning is usually good for about 20-25hp, and Hemifever custom tuning is usually good for about 30-35hp. With the Hemifever upgrade, you will also get custom transmission tuning as well.
As for intakes, the Vararam is going to give you the best performance gains with S&B being a close 2nd. You cannot go wrong with either of these kits.
And as for mileage, both will show mileage increases, but not enough to overcome the cost of the parts. Very few mods will pay for themselves from a mileage standpoint, but both mods listed above will add power and make the truck much more fun to drive.