Umm, the Petrel, Perdix, or Perdix AI you buy will not be controlling the CCR you buy, so it doesn't matter what you buy today.
If you want the computer you buy today to be able to accurately monitor a CCR, you only have one option, and that's the Petrel 2 EXT. However, the EXT is connected to a CCR via a Fischer cable, and there's a lot that can go wrong there, and it's extra bulk on the device.
BTW, to clear up some confusion, the Fischer port will monitor 3 cells accurately and the normal Shearwater voting logic rules apply. You'll also need to calibrate it separately from your controller. If you're diving a Shearwater driven eCCR (JJ, SF2, Prism2) you'll likely have a Shearwater connected via a digital signal cable called CANBUS, if you're diving an mCCR (KISS Explorer) you'll likely just have a Shearwater connected via a Fischer Port (there's no controller on the mCCR).
IMHO, if I was buying a computer today, and not sure I was going to go CCR in the future, I would just buy a standalone Perdix or Perdix AI. Even without the Fischer port, it can be used with a CCR; it won't monitor your PO2 in real time, but it will track your loading based on what you tell it you're running as a setpoint. In over 350 hours of CCR diving, I have found my stand-alone Petrel running in CCR mode to match very close to what a Shearwater connected to the unit was monitoring.
Last thing I'd like to suggest, is if you're in the North Florida / High Springs area the weekend of May 5th, join us for the CCR workshop at Cave Country Dive Shop. We'll spend a day in a classroom discussing CCR design, theory, and function and that should clear up some of the confusion. Then we'll spend a day diving different units.
Ken