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From my experience, 135 is too high. I pulled one last year at 120. After it rested, it was about 126 in center. The ends were perfect for those that don't like it too rare. The center was good for those who like it rare.
As others have said, having both is preferred. If I only had one, I'd go instant read. My thermapen has changed the way I cook everything, not just on the Egg. If strapped for budget, at least get the thermapop. But if you can afford it, go for the thermapen. Lighted background is best.
Did my first spatchcock turkey this year. 15 pounder on a large finished in 2 hours. It was done perfectly and even. Probably won't do a turkey on the Egg any other way. Although, the backbone removal was more difficult than on a chicken.