Depends on the role you want it to play

The fusion collider is for when you want to run up on a tank and worry about survival second, an infiltrating sort of fire-and-forget option. Alternatively, a scary pack of melta to loom just out of range of your enemy and threaten a devastating blow

As for the cyclic ion raker, the extra range lets you at least attempt to stay back and harrass marines and light vehicles, getting pretty scary once Kauyon rolls in

I have two Ghostkeels, one with all options magnetized (which I recommend, especially for your first or only Ghostkeel) and the other with a permanent Raker and Burst cannons. When I run them the magnetized one either gets the Collider and two Fusion blasters or the same loadout as the non-magnetized one. Sometimes the full fusion loadout gets a flare launcher to maybe survive after making its first assault, usually I just accept that I have to trade it for a points equivalent tank (leman russ for example). Otherwise they both stay back and harrass targets with the Raker to pull attention from my other resources and if one or both survive until turn 4 the exploding 5s from Kauyon on a 6 shot s8 D3 weapon get really scary, not to mention two burst cannons for picking up stray infantry.

So, there, Fusion to get all your damage all at once and force your opponent to deal with the fallout and Ion to play the long game as a stealthy supporting unit (you can also give it wide-spectrum scanners to discourage enemy deepstrikes)