I think everyone goes through this dilemma. The Flashpaq will give you a slight increase in gas mileage, but nothing major. Also, the Procal lets you straighten the wheel. Personally, I'm going with the Flashpaq once I have the money because I wanted the performance boost. It's not for drag racing, just wanting whatever little bump I can so that my cruise control doesn't need to downshift on steeper overpasses.
'09 X Unlimited, 4WD, auto, half-doors, soft-top
RC 2.5" Series II BB, JKS Quick Discos, JKS adjustable front track bar
15x8 Unique 297 (Soft 8), Goodyear MTR/Kevlar 35x12.50R15
5.13 gears, ARB front locker, Currie front shafts
Bushwacker flat flares, Rubicon rails with steps added