If you’re genuinely unsure, start with diesel.
Diesel is more deterministic. Change rail pressure, change quantity, change SOI — the effects are measurable and relatively linear. Gasoline adds knock detection, enrichment switching, cam phasing.
Market considerations: In most European markets, diesel vehicles represent a significant portion of workshop traffic. Check your actual car mix before you commit.
If you’re planning to do both: Diesel first, complete it to independent level, then add gasoline.
Diesel Fundamental is where most people start — the torque model knowledge transfers to every platform.