Focusing on the mathematical mechanics of the question, we need to examine the differences in properties between the options.
Before considering mithral, the details of a Breastplate are +6 AC, Max Dex +3, Armor Check Penalty (ACP) of -4, and speed of 20′ (assuming a base of 30′). Normally, if I was using a Breastplate for a character, I would actually use an Agile Breastplate, which has the same basic stats, but only applies a -1 ACP to Climb and Jump (likely the most common checks in a lot of cases). Full Plate is +9AC, Max Dex+1, ACP -6, 20′ speed. With a Dex modifier of +5, then your AC in these armours would be a base of 19 and 20, and your Touch AC would be 13 and 11. You’d suffer -4 or -6 to Dex or Str based Skills, and move at 20′ speed.
Mithral lowers the category of armor for the purposes of movement by one step, increases the Max Dex by 2, and reduces the ACP by 3 (4 for masterwork, which would be the case also for any magical versions). If you are trying to maximise AC, you’d likely also be using at least a Heavy shield +1, for another +3 AC, and another -2 ACP. a Mithral shield would reduce the extra ACP to 0. So, Mithral +1 with heavy mithral shield +1 would give AC of 25 and Mithral fullplate +1 with heavy mithral shield +1 would give AC26. Touch ACs would be 15 and 13. Your speed would be 30′ in the breastplate, and 20′ in the fullplate. ACP would be 0 or -2. Most other AC buffs would then stack the same on top of either of these options, although further increases in Dex wouldn’t help much.
As you can see, the overall differences are minimal between the two options, for a considerable difference in cost. The major difference is in the base movement speed, which may or may not be an issue, depending on if you are mounted, and how often you have buff spells like Haste or Fly available.
For comparison, you can look at the monster stats by CR, and you will see that even the Low Attack option will be hitting AC26 at least 50% of the time by CR12. By CR20, you would need at least another 15 points of AC to give the high attacks a 50% chance to miss.