Happy Home Designer looks ok. It really feels to me that it would be more suited as an eShop game rather than a fully-fledged physical game release - £5-10 sounds reasonable, but anything over 20 sounds a bit much, assuming that there literally isn't anything more to it than decorating homes. Not too excited about Amiibo Festival as it looks like Mario Party but without the element of ruthless competitiveness which made the previous instalments so much fun to play.
All in all, I'm a little worried that the amount of spin-offs is indicating that we might be waiting a long while for the next main instalment. Even though the series definitely feels more at home on handhelds and it's difficult to think how they can top New Leaf, of course I want to see an AC Wii U title. Who doesn't? And all we're getting instead are weird spin-offs. Boo.
So I want to come back to this because judging by the treehouse stream and assorted trailers, it sounds like HHD does in fact have more to it, enough to arguably warrant its full priced.
It's basically the mayor aspect tuned up. You have complete control over the town. Who you want, where they go and the main "city" area you work in is several tiers of places you can build like Schools, Hospitals, arcades etc
The home designing is also fairly involved. Beyond working with a theme (obviously), there seems to be less restrictions so you can put more things in and can customize the surrounding yard as well with all sorts of items and works. The villagers can learn how to use certain items based on what you do. There are more actions and reactions animals can do as well. Over time (presumably tied to how happy they are, I imagine) the villagers will want to upgrade their homes to larger houses (apparently with multiple rooms) that need to be filled.
There also still seems to be an aspect of collecting new furniture judging by amiibo card descriptions; presumably you wont start the game with all options and will collect them or buy them as it progresses.
Designing the homes is clearly the crux of the game, but it's not as simple as "ok just design these homes". There's a lot of villager interaction with a lot of new dialog to go with everything, and I wouldn't be surprised if there was more than even what they've shown. It's not just a low effort $40 thing like presumably Amiibo Festival is.
If I had to guess, they're using this as a big test bed for things they want to do or implement in the next proper animal crossing. The way you build up the city, the fact you can choose where villagers go, the greatly increased outside customization, new items, new ways for villagers to interact, the increased options for inside, ability to choose your skin color, sidling, etc etc etc
They even call out how you have more power working for the HHA then you did as Mayor, which makes me think in the next one you'll be able to get at least a reworked set of functions similar to this (ie: you probably wont be able to devote a ton of outside space to a villager, but more optiosn at what you can put outside and where; ability to unlock the PDA to place things around; ability to upgrade villager homes,e tc). Especially with how hard they're seemingly pushing Lottie, the pink otter you work under/with.
e: In comparison Amiibo Festival is just sort of...there. Unless there's a lot more than meets the eye it's pretty light. Also I think I only ever saw one tweet saying it'd be free to download and I dont think it was confirmed by NoA staff or press releases or anything...