If I pay to upgrade Gigapixel will it also upgrade Photo AI? And vice-versa - will paying to upgrade Photo AI also upgrade Gigapixel?
Want the latest versions of everything, but want to know what I get before I pay for either.
Thanks!
Upgrading Gigapixel will not Upgrade Photo AI. 
But will upgrading Photo AI upgrade Gigapixel?
Denoise and Sharpen are not available any longer and their links direct you to PAI. I own all four and still use Sharpen and Denoise but they are both really end of life.
If I had to pick only one between PAI and GP at this point it would be PAI. It has the enlargement feature and you can stack several other functions on the same image if required/desired.
