Do metal enclosures block radiated emissions better than plastic ones?
Yes it will (for most).

The enclosure will act as a "Faraday Cage", as the enclosure is conductive, any EMI emitted will hit it and will be absorbed/reflected, it can also stop RF.

Although the case will not stop EMI emitted through the supply lines or if you have some other cables going out, you would have to filter those properly as any cable going in / out act as antenna.

If you have a high frequency power converter, it can generate noise and push it back to the supply lines.

The case needs to be grounded, for safety reasons.