First of all, CONGRATULATIONS on painting your car. It looks like a great job! I have never yet had the balls to paint a car, so my hats off to you sir! Well done!
Now, to paint differences: I agree with some other comments that that two gallons shouldn't be that far off. Here is something to consider: How much work is it to repaint the under hood areas? I know it is extra work and a pain in the butt, but it is still relatively easy before you bolt stuff in the engine compartment. If it is going to bug you every time you pop open your hood, bite the bullet now and you'll be glad you did later!
Regardless of what choice you make, congrats again on a great job!