If you're looking to keep the ATP and proceed to an upgraded class of machines, I'm thinking (though I've never used one) that the XP may be easier to learn than the CTX - if you're focused on getting into the guts of the computer you have in your hands. I knew my CTX had complexities I hadn't explored, but had no idea how much until I attended the Andy Sabisch FBS boot camp.
Basically I used mine as a beep & dig machine just listening to the sounds from the boot up screen, I'm still a sound guy. Really, from the time that I put in with the CTX I feel like I did very well.
So with just the simplicity of just turning it on and digging - that makes the CTX just about as easy to use as the ATP.
I've also hunted along side an XP, an old TNet member who passed away this Winter - Meteor Man. He'd been detecting since the 50's, so he understood the basics. And of course I'm a lot slower than anyone else I hunted with, so everyone else always kicks my butt. But I'm methodical, doing my own thing and not minding the butt kicking I receive
I guess in my evolution with the CTX taking just the simple approach just actually identifies the basics of metal detecting = there's no pleasure in the world like a solid signal in two directions! Simple as that!
So, really just do what feels simple to you, is based on what you've learned so far, and certainly suits your curiosity. I've always believed in knowing what's out there and the capabilities there of. Do you want to stay in the Garrett family? You'll do well there, always competitive. But does your curiosity demand to know everything else outside the realm you're used to?
Keep the faith in whatever you choose. As long as you enjoy the hobby all is well.
But, another point. I think I've owned about 19 machines now since '81. That was a whole lot of money I've spent over the years. I don't think I'll ever get repaid that by the treasures I find. I was carried away by the curiosity I was absolutely enjoying.
Plan your life better than I did!