The Sorrow vs Theodosia Electra: who would win?

The Sorrow favored — Slight edge (~55%)

The Sorrow · Metal Gear · Base

Tier: 8-B
Attack Potency: City Block Level
Speed: Superhuman, up to Massively Hypersonic
Durability: Unknown

Theodosia Electra · Toaru Majutsu no Index · Base

Tier: 8-B
Attack Potency: City Block Level
Speed: Average Human
Durability: Human Level

A heuristic estimate from normalized VS Battles Wiki stats: tier, attack potency, speed and durability.