Hiroto Soga vs Rei'sen: who would win?

Rei'sen favored — Slight edge (~55%)

Hiroto Soga · Kagurabachi · Base

Tier
8-B, up to 7-C
Attack Potency
Town Level
Speed
Massively Hypersonic+
Durability
City Block Level

Rei'sen · Touhou Project · Base

Tier
8-B, up to HIGH 6-A
Attack Potency
City Block Level+, up to Multi-Continent Level
Speed
Infinite
Durability
City Block Level+, up to Multi-Continent Level

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