So from what I can see Avast One is the same as Avast Ultimate except it also can:
"Clean up, speed up, and update your devices" By this I infer it includes Driver Updater, which isn't mentioned in Ultimate???
Ultimate has Antitrack, so I'm assuming it can do the bit that One can also "Get an alert if any of your passwords are leaked and secure your accounts"
But yes, I also cannot find a simple comparison for both. I'm assuming otherwise they're basically the same.
I couldn't even find a link to Ultimate from the Avast website, but did via duckduckgo. It exists, I just can't seem to navigate TO it from the main Avast site, even though it's is on it, and I CAN navigate FROM it back to the main site.
I only found out about Ultimate when I contacted Avast asking to roll subscriptions into one and upgrade to 2 or more devices as I had just bought a laptop. Seems weird they wouldn't be promoting it like crazy.
Ultimate refers to being for 1 PC $64.99 or 10 devices at $79.99 a year (I'm in Australia and it looks like there's a discount atm down from $133.99/$167.99), and Avast One is for 5 or 30 devices ($70.68/$95.88 down from $139.99/$189.99).
Seems to me that One is the better deal.