Makes no difference to the licence you will get. The difference between the two exams really only lets you choose to do a more relevent exam to the type of operations with which you might be most familiar.
Much of each exam is common. Neither is more difficult than the other. I chose to do the 121 exam when I converted my Oz ATPL. No great reason, just thought it would be interesting.
No matter which exam you do they're easy compared to Oz or JAR. Buy the Jeppesen, ASA or Gleim book(s) and study on the flight to the US + a few nights before you depart. Free practice exams are available online at a few websites and worth doing. The US has all its questions publicly available so no problem for people to publish them.