Magic Resist options:
Abyssal Scepter
Aegis of the Legion
Athene's Unholy Grail
Banner of Command
Banshee's Veil
Chalice of Harmony
Guardian Angel
Hexdrinker
Locket of the Iron Solari
Maw of Malmortius
Mercurial Scimitar
Mercury's Treads
Mikael's Crucible
Negatron Cloak
Null-Magic Mantle
Quicksilver Sash
Spectre's Cowl
Spirit Visage
Wit's End
Zz'Rot Portal
That's 20 options. We have MR on each of these items, mixed with:
- Magic resist reduction
- MR aura
- health
- CDR
- mana regen
- magic immune minions
- AP
- AD
- armor
- shield passive
- shield active
- CC cleanse (one on self, one on ally)
- increased healing
- AS
- health regen
- MS
- spellshield
- resurrection passive
- zz'rot minions
I'm struggling to see a single thing missing from that list.
Meanwhile for armor:
Chain Vest
Cloth Armor
Dead Man's Plate
Frozen Heart
Glacial Shroud
Guardian Angel
Iceborn Gauntlet
Ninja Tabi
Ohmwrecker
Randuin's Omen
Raptor Cloak
Seeker's Armguard
Sunfire Cape
Thornmail
Warden's Mail
Zeke's Harbinger
Zhonya's Hourglass
Zz'Rot Portal
That's two fewer options at 18.
We have:
- Health
- MS
- AS slow
- CDR
- resurrection passive
- MR
- Mana regen
- Spellblade
- Critical strike damage reduction
- slow
- permanent AOE damage
- Auto attack damage reflection
- crit / AP bonus
- stasis
- zz'rot minions
Basically, all of the same stuff. Logically swapping out some spell protection (in the form of cleanses and spellshield) for some AS slow / crit reduction / stasis.
Two things are missing from the armor set though: minion buffs and an aura item. Add two armor items and they're basically the same options (well, actually, the armor items only have Iceborn Gauntlet doubling as an offensive item, there are many MR items that also give offense).