I can assure you it won't be USB-B ...
IMHO it's a toss up which way they go. One could easily argue for either:
- AW power needs are minor and nearly everyone has a bunch of existing USBA power outlets/adapters available, so a USBA cabled puck would have the most widespread compatibility with people's existing charging options.
- On the other hand, with the Apple Magic Keyboard & Magic Trackpads now shipping with braided USBC to Lightning cables it's possible Apple is beginning to transition to shipping peripherals with USBC cables as the default.
Either way they go I'd expect the other option to remain available for sale if needed. I'd also hope they'll continue compatibility with prior charging pucks.