I'd have to say the inferno if I could only have 1. The problem I have with the Ion is that the guns are not changeable. And that slow fire rate will effectively make it ineffective against smaller ships. It's like take 6 shots in 10 seconds, miss 5 hit 1. I know they are design for bigger ships but the infernal could definitely go against smaller ships as well with the high fire rate. Maybe I'm just a bad shooter lol
In addition to this. ION lacks large radar...Against large and capital combat ships can be destroyed in seconds if enemy radar locks it before. Keep in mind that eclipse which performs the same role with torpedoes (instead a large cannon) is a stealth ship.
I think Ion and inferno will work as an
auxiliary chivalry, to use it as a second wave attack to turn the fate of the battle in your favour.
They must be hidden ships (in a near asteroid belt or the blind side of a moon) ready to punch when enemy forces and their primary batteries are busy engaged with same size ships.
Then as second wave fleet, inferno can wipe out quickly damaged & out-of-ammo fighters and remove them from the battlefield, and ION can create a breach in the capital shields enabling a decisive advantage for friendly capital ships heavy fire to finish the job.
Also can serve as a diversion in order to divide the enemy's fleet, if you show deliberately your Ares ION wings, probably an inexperienced admiral could send several wings of fighters to chase them, leaving the main group defenseless.