I will offer a partially informed opinion (as always)! I think it is just that the groundschool that you do for the FAA course is considerably lighter than the JAA equivalent so I reckon most of the time saved is as a result of that. The JAA groundschool involves 14 massive binders full of stuff, while the FAA one is one book I think?! Which is better? Well, I know which one would have been easier!
The flying as far as I am aware should take about the same length of time.
Don't forget though that you don't get an ATPL (or ATP). You get a commercial licence and IR which you can convert into an ATPL (or ATP) when you have 1,500 hours and the additional requirements. You need to do a checkride for both upgrades and maybe do the FAA ATP written unless you can do it earlier - not sure.
FAR 61.153 and LASORS section G 1.2
Hufty.