DEX-focused heavy armour user – mithral full plate vs mithral breastplate

So I’m curious about a matter of best AC for my buck. I’m playing a Dexterity focused heavy armour character (Cavalier to be specific, but I imagine the same answer applies to fighters or any other heavy armour proficient characters), My starting Dex was 16, so without taking into account a Belt of Incredible Dexterity my DEX by level 20 will max at a +5 bonus.

I’m trying to decide whether I am better off using my heavy armour proficiency, getting Mithral Full Plate and accepting that I can onlyuse +3 of my DEX bonus for AC – though I believe I can still get my full DEX bonus to other things such as Weapon Finesse and Deft Strike (when I multiclass into Swordlord later on) and Reflex saves.

Or am I better off with something lighterweight like Mithral BReastplate, even thought he base AC is lower, get to apply more of my DEX to my AC and not take the movement speed penalty for wearing Medium armour which I would for having Full Plate.

I’m not especially confident in my math as to which can get the better overall AC – as I am a front-line tank and really my ideal situation is to maximise AC over any other concerns.