Chromecast Ultra  is my third Chromecast. It retails for US$69 (S$99). It doesn’t ship to Singapore directly, so you have to go through a freight forwarder like HopShopgo.
I bought my Chromecast Ultra from Google Store (US) . You need a US VPN in order to access the site and buy items from there. It will ship to freight forwarder addresses and it does accept Singapore issued credit cards.
Chromecast Ultra is not a successor to the Chromecast (2015) . It is a another Chromecast product line. Chromecast Ultra as the name sounds, is a high-end model Chromecast.
Chromecast Ultra supports 4K streaming and comes with the an Ethernet Adapter. Previously, to use LAN, you need to get the optional Ethernet Adapter for Chromecast  which costs US$15 (S$22). I always opt for wired connection as far as possible because the buffering difference is noticeable.
Because the Ethernet Adapter is also the power source for the Chromecast Ultra, you cannot use your TV USB port to power it. Your TV USB port doesn’t have enough power to power up the Chromecast Ultra. I tried and it doesn’t power up. If you did not use the included Ethernet Adapter to power up your Chromecast Ultra, your Chromecast Ultra will default to 1080p stream instead of 4K stream.
It measures 58.2mm x 58.2mm x 13.70mm and weighs 47g. It is just slightly bigger and 8g heavier than Chromecast (2015).
It also supports both the 2.4GHz and 5Ghz bands with Wi-Fi 802.11ac similar to Chromecast (2015).
Another cosmetic difference between Chromecast (2015) and Chromecast Ultra is that Google dropped the Chrome logo from the Chromecast Ultra and replaced it with the Google logo.
I would recommend getting Chromecast Ultra if and only if your television is Ultra-HD 4K ready as it is future proof. If your television only supports full HD, there is no point in getting Chromecast Ultra.
If you need an Ethernet connection for your Chromecast because of slow Wi-Fi, do note that it is cheaper to get Chromecast (2015)  at US$35 (S$50) and the optional Ethernet Adapter for Chromecast  at US$15 (S$22) then one Chromecast Ultra at US$69 (S$99).