Stadia Pro is promised to deliver 4K/60fps with 5.1 surround sound, while the Stadia Base service only offers 1080/60fps. It further notes that "Stadia Founder's Editions and Stadia Premiere Editions will begin shipping in the same order that pre-orders were received".
Stadia Pro subscriptions net members additional games for "free", starting with Destiny 2: The Collection. Restricting wireless play to the Stadia Controller (and only allowing the Chromecast Ultra to pair with the Stadia Controller rather than supporting additional devices) inhibits the platform's ability to deliver on that experience. Instead of connecting via Bluetooth, it matches up directly with Google's Stadia servers over Wi-Fi and sends the signal back to whichever device the game content is being streamed from. The announcement of launching PlayStation' competitor came in March this year during the Made by Google 19 presentation. However, if you believed that the controller will always be wirelessly attached to the screen, you are mistaken. More phones, including the iPhone, and tablets are slated for later.
You can watch the Google Stadia 101 trailer below.
Wireless controller support was demonstrated live on-stage during Google's GDC show across multiple devices, so it is rather unexpected to hear this feature has been pulled so close to launch later this month. Rather than simply moving from playing on a TV (via a Chromecast Ultra) to a PC or their phone, players will have to take the additional step of plugging in a USB cable.