If it were me, and the current cartridge was well taken care of and in great shape, I'd opt for the better protractor. However, you need to be sure you're up to performing the task correctly.
I have used a Geodisc before when I had my SOTA Star Sapphire, Kuzma Stogi arm, and Benz Glider. The Geodisc was easy to use, but the Wally Tractor I subsequently purchased did a much better job of allowing me to set up things properly.
I have used a Geodisc before when I had my SOTA Star Sapphire, Kuzma Stogi arm, and Benz Glider. The Geodisc was easy to use, but the Wally Tractor I subsequently purchased did a much better job of allowing me to set up things properly.