Piggyback is free (as far as the FAA are concerned) and really easy. The only 'hassle' is that you need to actually go to the US to collect it - but just plan that in for the first day you arrive (when you wouldn't want to be flying anyway).
I'm now doing my FAA IR on my piggyback licence, no problem.
If your licence is UK CAA based then you need to send them GBP18 (I think) - er, obviously??!!
Sam.
PS a couple of points, you will need an FAA medical, and to fly a biennial flight review to keep yourself FAA legal.