Basically from what I know, LT's will give more Hp but likely not pass emissions in many states. Shorties will provide more low end Tq and will always pass emissions. Mids will give you a balance of both. But the LT's are best overall especially since they will really compliment other mods.
yah kb112 really nailed it. i have the longtubes and love them, though they require that you do some mods to keep the cats, or ditch them like i did. and if you do that, expect it to be loud. i like it though, most of my family doesn't.