To be honest, the class isn't granting you a lot.
As an arcane caster, you can use heavy armour at the loss of your swift action, but if you don't take a dip you're burning three feats to enter. Reduced ACP for your armour, too, but who cares? Armour's not even that good anyway. I like Assiduous Gaze and Detect Lies has RP/social potential, but it's not worth losing your Hexes (as a Witch) for, let alone spending the feats on. For a Wizard, maybe. Infernal Resilience is... not useless... and Diabolical Harbinger is cool too but you've sunk 10 levels in by then. The other thing you're gaining is BAB, but it's not a big difference until the higher levels. If there is some archetype which grants armour to one of those classes, perhaps - but entries like Bard stand to lose even more and gain even less.
As a divine caster, you're not really standing to gain very much at all. You can already cast in heavy armour, probably have medium BAB, etc. However, a Cleric only needs to spend one feat, and it's a bit of a dud but not useless, and Hell Knight progresses domain powers and Channel, so you lose nothing really. You also get nothing out of the ACP reduction or any of that, but those other bonuses are a little something on top of what you keep from your class.
I think I'd have to agree therefore that Cleric works better; going into Hell Knight means you're pretty much forgoing Bloodlines, Schools and Hexes. Wizard or Witch would probably be for flavour only (or because Wizards are just better
) or if you wanted to make some kind of arcane gish that was a bit less boring than Eldritch Knight.