I own and enjoy both, and they're two models that really showed what Cold Steel and Andrew Demko could do when they were firing on all cylinders. That said, they both have pros and cons. The AD10 is an absolute brick in the pocket, with what is essentially the door from a small office break room microwave bolted to it as a pocket clip. The AD15 is slimmer and takes up less pocket space, but unfortunately weighs roughly 14 lbs, and the handle is very oddly unforgiving in the hand with its hard angles despite the finger choils. However, both are screaming sharp, and feature big stout blades of CS's well done S35vn. Neither of them has ever failed to cut anything I needed them to cut, and both lock up like bank vaults.
So, given that, if I had to pick one, it would be the AD10. It fits my hand absolutely perfectly, and is a joy to use, whereas the AD15 is a bit more of a fidgit toy with the excellent Scorpion lock, but not all that comfortable in the hand. Ultimately, I enjoy and carry both on occasion, but candidly, I tend to really only carry them on days when I'm out in the woods, or camping, working in the garage, or some other activity where I've got cargo pants on. Most days, I carry other knives instead.