Exhaust won't gain you anything until you're more heavily modified. Go with an intake system,(air box/filter), 4 or 6# lower pulley and a tune. This will be the best bang for your buck. www.nloc.net has a ton of info too.

But for the record, I will put down the SRT is faster out of the two off the dealer floor.

The SRT-10 is not a hands down faster truck stock for stock. The SRT's are a 6 speed that 98% of the owners can't launch to save their a55es. And if the SRT doesn't beat the Lightning out of the hole it will lose every time. The Lightning stock is  very easy to launch and being an automatic, you won't be missing shifts. Ask the SRT guys how fun it is to find 3rd on a speed shift. This is why virtually every SRT-Lightning encounter on the street/track ends with the Lightning  as victor. The SRT 10 is a nice truck, but not for the price. Not to mention the cost of anything to mod it.
   I've ran 2 different SRT's with my old Lightning. It was a 2002 stock down to the filter lightning and I won both times by a considerable margin. I don't even bother with them now with my new Lightning as it's heavily modded and it'd be silly to waste the gas on a stock SRT-10. The chance of finding a heavily modded one to run on the street is slim to none. Most owners won't race around here anyways.