Exhaust preference is highly subjective, in large part due to the way different exhaust systems sound. You'll be reaping nice benefits by uncorking the factory exhaust on your 03/04 - no doubt about it.
Try listening to a few different exhaust setups in person. Check out your local mustang meets/gatherings to help get a better idea about how the different exhaust systems sound before you drop your hard earned cash on exhaust parts.
Oh, and you can also try the Terminator FAQ (sticky thread at the top of this forum - direct link to it: Terminator FAQs ) - there should be some helpful Terminator-specific exhaust information within...g'luck.
one thing too is if you have some qood quality speakers is check youtube or streetfire or some video based site and put in the exhaust system your looking at. thats what i did and you can get a good idea. granted it doesnt do justice as in person but you can kinda get the feel for it.
So many setups and variations you can do. I loved the sound of MAC pro-chamber with Borla Stingers when I had it on. Switch it out once I went SRA. Have yet to put on LTs because of the hassle. Figure I will once I switch out for an aftermarket K-member and such.
Many vids on youtube to check as stated above. Good luck
I'm a Magnaflow fan. Had Flowmasters on my 97 Cobra and Magnaflow catback on my 99 and 03 Cobras. Not so loud that they are annoying, just deep and rumbly enough to say "back off before you get smoked".