Stingers...there, that ends that discussion.
O/R X? Well, no they are not legal. It will require either MIL eliminators (which I believe companies are not allowed to sell anymore) or a custom tune that disables the rear O2 sensors or else you will have a permanent check engine
light. Also, if you require yearly inspections where they use a sniffer device on your exhaust, you will not pass the test. If they only plug into your computer to read the codes like they do here, then you will pass fine as long as you do the tune or MIL's and as long as they don't look under your car.
Why do we put them on? Well, the sound is the first reason. An X-Pipe will give you a bit more exotic sound than an H-Pipe would. It definitely gives you more volume as well. You will also find that you will gain a little power from an off road as well because it frees up the exhaust flow and improves exhaust scavenging ... so it improves the efficiency of your motor = a little more power.
Now...I guess back to the exhaust. Borlas will be plenty loud for most even with the stock cats and stock H-Pipe. So, if you replace your H-Pipe with a catted version, you will still be plenty loud. It will be nice and mild when cruising, but once you press the loud pedal, the volume will go up quick. But, you won't go wrong with the stingers. They have a great muscle car sound to them. Avoid the chokemasters like the plague unless you do a 5.0 swap...because unfortunately, they sound pretty good on 5.0s.