If you'd like more information, pick up a copy of Your Packet Companion at your favorite amateur dealer. First, let's not confuse the terminology. AX.25 is a protocol that defines the bits that are sent. It depends on the particular characteristics of the FSK and PSK systems being used. In these tests, PSK proved superior.
Ionospheric effects complicate the picture.
A modem that works well on a solid, fadeless channel may not work well at all once fading begins. In the presence of fading, it may turn out that the modem that works best is one that didn't stack up so well against other modems when tested on a fadeless channel. All of which means that there is no one answer to which kind of modem is best. Indeed, the push these days is for adaptive modems, modems that change their characteristics to adjust to changing propagation effects. With regard to protocol, which is a separate issue, as noted, I don't think any knowledgeable person would argue that AX.25 is even close to being an efficient protocol for HF work.