I remember a debate at Keypubs forum a long time ago. Some of the western commentators claimed the 3M-54E/TE was deadlier than the ASCM with all supersonic flight profile. The main subsonic stage will make the approach stealthier (especially the IR signature) and will allow it to sneak up on a ship before releasing the supersonic final stage. This should apply to the YJ-18 as well. The YJ-12 advantage might be the size of the warhead and overall range. With a 300 to 400 km range, a platform like H-6 can stay away from most ship-borne air-defense systems.
From what I can understand is there is also the difference of mid phase guidance.
The YJ-12 being a ramjet, achieves its highest fuel efficiency at high altitudes and high speeds. You can't get the maximum range with a low low and low approach. This makes the missile to spot and easier to defend against. The flip side of it, is that a ground station can continue to send guidance updates to it, without losing track of the missile if the missile goes low in mid phase.
The YJ-18 has an initial turbofan or turbojet stage, followed by a terminal rocket stage. Thus it can sea skim at long ranges for a low low low approach. The problem with that is that the missile will lose contact with a ground and surface station, and to continually receive mid course updates, you need to hand over control to an airborne station. But what happens if the airborne asset falls under attack? If the PLAN is so willing to invest heavily on the YJ-18, then its confident that it has operative cooperative engagement, that it the airborne assets to back it up and protect the eyes in the skies.
The YJ-18 can also do a high to low approach so it can continue to receive updates from a ground station but it loses all the advantages of its design versus that of the YJ-12. Being much slower from point A to B, the YJ-18 faces greater risk of interception compared to YJ-12. YJ-18 is likely in my view, goes low all the way, and that's the justification to its very design.
Lacking an OTH radar on the 51B refit, means there is no point for the ship to be a surface station handing out updates. The ASMs it fires has to be cooped in mid air to an aerial asset. That kind of makes me lean now to YJ-18.
There is nothing that says YJ-12 can't have cooperative engagement, with control transitioned to aerial assets. But when you have 10 YJ-18 + 10 YJ-12 in the air, I don't think its better than having 20 YJ-18s. Its much easier to coordinate a whole bunch of the same missiles in the air, line them up into a swarm or in synchronized waves of attack. The thing about the YJ-12 is that due to its speed, it will get to Point A to B faster than a YJ-18, and that makes coordination impossible as the YJ-12 gets there first. The defenses would already be alerted early of the YJ-12 approach, and then would be ready for the second wave of YJ-18. Whereas if the launch waves are all YJ-18 from all these different ships, it would be easier to synchronize and "wolf pack" the missiles.
I might also like to add, this also makes coordination with the older YJ-83s easier, as these are also subsonic in mid phase. The main difference between the YJ-18 and YJ-83 is how they handle the terminal phase, with the YJ-18 boosting into supersonic speeds while the YJ-83 remaining high subsonic.
While the refitted Sovremennyy may still have its OTH radar, there remain these advantages of going all out YJ-18.
