Tabi/Mercury should be able to trade stats between each other.

(What we would OFTEN want) +30 Armor +30 Tenacity

or

(Yes, I realize this option will barely be used, just add it for consistency) +25 MR 10% Basic Attack Dmg Reduction

Like this. It should be a choice of a passive-item. The items are very SIMILAR to each other. It feels horrible when you're against almost full, ad but you need tenacity, so you buy MR boots when what you really want is armor. Or you want to buy Tabi against one ADC, but it's really just one ADC, rest being magic damage, so you want both MR and basic attack dmg reduction (I realize the latter option will be less popular, still, for consistency)