Hi Tom. I hear you're busy with the Pitts training! I hope it's going well Gosh, it's hard to say which is best for you to start in: Primary or Sportsman. If you have no previous aerobatic experience, and especially if you're flying an aircraft which is more difficult to fly aerobatics in (eg Citabria), perhaps Primary might be best.
However, if this isn't the first time you've been upside down, and considering you have a Pitts, which is pretty easy to fly aerobatics in, you might consider starting in Sportsman.
Heck, I know someone who started in Intermediate, and actually won the Unknown flight at his first contest!
One big difference between Primary:
and Sportsman:
is the length of the sequence, and the challenge of staying oriented, and correcting for the wind, during the longer sequence. The maneuvers aren't that much harder in Sportsman. I think the big difference between the two this year is that there is a spin in Sportsman. IAC is very conservative with spins and rolls - you will only see 1/1.25/1.5 turn spins, for example, which actually don't allow the spin to fully develop.
It's really a matter of what you feel comfortable with, and how much practice you get in, and how much critiquing you can get - don't practice it wrong!
Every person is different. It really doesn't matter that much, to tell you the truth. Don't be in a hurry - enjoy the trip, it's a wonderful one!