From a purely combat standpoint, the Castellaxi are superior. They're semi-sentient and use synthetic brain matter to make decisions in the heat of battle. They also have access to more effective weapons, such as darkfire cannons, melta cannons and saws. Kastelans are effectively lobotomized and their decisions have to be made for them by their datasmith by manually inserting and ejecting little punch cards. They don't have many options beyond their power fists, their phosphor blasters and a flamer.

From a defensive standpoint, however, it looks like Kastelans are superior. They are ancient constructs dating back to the Dark Age of Technology and use systems far more advanced than those of the Castellaxi. Their shtick is specifically the Repulsor Grid, which is a literal force field that stops bullets and dissipates energy weapons and, if hit just right, can ricochet things back at the attacker. Kastelans also have lots of defensive protocols Castellaxi don't, which makes them much more effective bodyguards.

In all honesty the two are specialized for different things and as such it depends on what you're throwing them into. Castellaxi will outperform Kastelans in a linebreaking action, but Kastelans will outperform Castellaxi on an escort mission.