It depends on what you like doing really, the Python fully fitted for combat still easily gives 200T of cargo space, so if you do any trading go for the Python.
If your time if fully combat based and you have no interest in trading then the FDL may be more up your street.
Ammo isn't that expensive, the issue I have is that the cannons and PAs are best at close range, fine for large targets, but not so good for smaller ones.
I haven't tried the FDL, but I can say that a fully upgraded Python is great for combat.
That said if you have the funds to swallow the 10% loss of selling a ship you can always try one and then the other (remember to sell/down grade your modules first)
Edit: well and truly ninja'd by Loriath