propane will add power and can increase fuel mileage, when combined with proper programming. however if abused it will break stuff. it can be really rough on rods and headgaskets.
nitrous won't do anything unless you've got larger injectors and don't have enough turbo to clean up the smoke/unburnt fuel.
i personally would go with(have) a set of beans stage 1's or with swamps "baby swamps". these will give you the powerboost and since they use stock nozzles they can be turned down for towing, while still making more power than stock. these also should have a few other supporting mod's, ex. intake, exhaust, gauges, and a tranny/clutch depending on tranny type.