Should I favor the +20% affinity or the type's research?

They work together, but here's how to decide:

• A big research gap between two types → research wins: take the most researched type, even if a few dragons end up neutral. Research is a permanent multiplier on the whole army.
• A small research gap → the +20% affinity decides: pick the type you have the most green dragons for.

The ideal combo is when both point at the same type: your most researched type IS also the one your roster is greenest for. That's where you stack research × affinity on your best dragons.

All else equal, remember the order of the levers: type research ≥ Center aura > +20% affinity ≫ ±7% matchup.