X & H pipe vs straight dual exhaust

When looking at exhaust I see a lot of x-pipes and h-pipes and was wondering what advantage this gives you if any or is it just a sound changer. I have always just gone straight back to a muffler. I will be using TTI headers and will be running on the street about 1500 miles a year. They sell both x & h pipes that will fit on my 66 dart but it's an expensive option! Has anyone gone from a straight exhaust to one of these pipes and seen a big difference?
Thanks
Rodney