Go with the 10-gauge.

Get the 10-gauge 30-amp cord. Don’t mess with adapters on something that can start a fire.

Thicker wire handles more current with less voltage drop. If your generator can push 30 amps, a 12-gauge cord is undersized even with a twist-lock adapter that physically fits. Adapters don’t magically make the wire thicker — they just change the plug shape. You’re still limited by what the cord can carry.

A 12-gauge 20-amp cord will get hot under a 30-amp load. Voltage drop also means your tools or appliances might not run right, and you could damage them. The 10-gauge cord costs a little more but is the right tool for the job. No adapters, no guesswork.

The only situation where the 12-gauge makes sense is if your generator’s max output is 20 amps. But if you’re asking, you probably want to be able to use the full capacity down the road. So buy the 10-gauge once.

This is not where you save twenty bucks.