In reply to Marcus Tibesar.

Yes, that's right. If the theme is AMP-compatible then Standard mode is best because it means desktop visitors get the same UX of AMP as mobile visitors.

However, there's still a case where Transitional mode may be preferable: if you're using a plugin that is not AMP-compatible where some of its functionality has to gracefully degrade on AMP pages (due to the custom JavaScript being stripped out), then in this case Transitional mode can be better since mobile visitors would have the opportunity to navigate to the non-AMP version to access that functionality.

The upcoming plugin compatibility scanning feature in the AMP plugin will help guide you when Transitional mode is preferable to Standard mode for your site.