The difference b/w both Chromecast devices is that Google TV one
- Comes with a Remote & doesn't need phone to run
- Has Playstore Pre-installed
- Can stream 4K-HDR content
- Can route audio to your bluetooth headset
- Has marginally better performance overall
If you've a 4K TV & don't want the hassle of controlling your TV with your phone, then get one with the Google TV.
For simple chromecast, everything has to be sent via phone. If this & lack of 4K doesn't bother you then save money & buy a 3rd Gen Chromecast.
*There's another discontinued Chromecast Ultra that can also stream 4K content but doesn't come with a remote.
I'd personally go for Google TV Chromecast because playstore access is a big positive. There's a modded Youtube app that can be installed on it easily which removes all the ads.