Just to make sure everything is crystal clear...
The Hubitat Community created HADB integration, allows HA devices/entities to be brought into Hubitat Elevation. The communication between the two systems is fully bi-directional. In other words, Hubitat could control a light switch device that only exists in Home Assistant. This integration requires no custom code on the HA side of things.
The HACS-Hubitat integration, allows HE devices/entities to be brought into Home Assistant. The communication between the two systems is fully bi-directional. There is no custom code required on the Hubitat side, but one does need to install and configure an instance of the Hubitat Maker API app on the HE hub.
It is important to no create a loop using the above two integrations at the same time. This could result in a very messy situation! 
As for the big differences between HA Yellow and Green...
- the cost of the HA Green + SkyConnect Zigbee USB Dongle is $100 + $30 = $130
- the cost of the HA Yellow + comparable RPi CM4 (4GB RAM, 32GB eMMC) module is about $194 for the nonPoE version, or $205 with PoE
The HA Yellow, as previously mentioned, already includes an onboard version of the SkyConnect Zigbee module. As @672southmain mentioned, one can also easily add Z-Wave inside the HA Yellow's case. I have the Zooz ZAC93 module happily tucked inside.
The other big advantage of the HA Yellow is the option for using an NVME solid state drive. I personally boot my HA Yellow directly off of a 1TB NVME drive. This means no wear and tear on a smallish 16-32GB eMMC drive. I personally run multiple HA Add-Ons, like InfluxDB, Grafana, Node-RED, NUT, etc... on my Yellow. Having an NVME drive brings peace of mind that all of the constant writes generated by InfluxDB are not going to wear out a smaller eMMC drive.
Finally, depending on the RPi CM4 module you decide to use on the HA Yellow, it is possible to use Bluetooth and/or WiFi. I would personally not recommend using WiFi to connect a HA Yellow to your home network. Ethernet is a much better choice. Neither Bluetooth or WiFi are built in to the HA Green.
The HA Green is trivial to get up and running, though. Plug in Ethernet, plug in power and then you're up and running. The HA Yellow, if bought as a kit, requires a little more work to get HAOS loaded on the board, especially if you're trying to boot directly from an NVME drive. It is not difficult, but also nowhere near as plug and play as the HA Green.
Both devices support HA's implementation of Matter. Matter uses IPv6 behind the scenes for all device communications. Matter over Thread does require a Thread Border Router device, which HA can sort of achieve using the SkyConnect module. But please do not try to use that module as both Zigbee and Thread at the same time. Most users have reported problems over the past year attempting that. Best to keep your Zigbee radio separate from your Thread radio. If you have a recent Google Home hub, Amazon Echo 4, Apple HomePod Mini, or Apple TV 4K (certain models), then you already have a device that can take on the role of Thread Border Router. This is exactly how Hubitat's Matter implementation currently works for Matter over Thread devices.
Hope this helps... probably more than you asked for, but I figure this might help others trying to make a similar decision.
I use Hubitat for my Zigbee devices, Lutron devices, Philips Hue devices, and all of my Rules/Automations. My HA Yellow is used for integrating devices that cannot be added directly to HE, and also to run other always on services like InfluxDB, Grafana, and Node-RED. Having both is really the best of both worlds. Good luck and have fun!
