Count the number of sprockets in the cassette. Thats the main thing, knowing if you need a 10, 11 or 12-speed cassette (speed is the number of sprockets)
Then you can also count the number of teeth on the largest sprocket - its also often stamped onto it, but might be hard to see if the cassette is a bit dirty. Mostly this won’t matter for the trainer, but if you have something out of the norm, for example a MTB with a 52t largest sprocket, your chain might be too long or too short if you have a more normal road range cassette (eg 11-30) on the trainer. (You can also count the teeth on the smallest sprocket to be extra sure, but it is most likely 11, with a small chance of 10 or 12 - the difference in chain length is most likely marginal).
That is up to the user if it is better or not. It really depends upon your use case - do you use multiple bikes on the trainer / share the trainer with a spouse / room mate / sibling / child? If so, then the Hub One is a probably a better solution. If you only use 1 bike, then it is probably a wash
TR doesn’t have virtual shifting. The virtual shifting is a zwift feature. However, if you only ever us TR in erg mode, you don’t need to shift, and then the one cog is enough even for TR.
If in doubt, I’d go for the version with a cassette for more versatility.
I would note that using one cog for TR may not be sufficient, depending on your trainer. Some have “floors” and “ceilings” for certain gears and you may not be able to achieve the target power if yiu only have one cog.
On my old Elite Direto (and Direto X), I ran into this issue regularly.
Ok, I confirmed the Hub is only one BLE channel. So you need to use (or add) an ANT+ USB stick on your laptop to pair to TR, since I’m guessing you use the BLE for Zwift.