I have a second generation Chromecast on an old TV that only does 1080p. I'm fine with that, I don't care about the resolution. Someone told me that I might like the new one with Google TV, but I don't know much about it. I pretty much just cast from apps on my phone, so I'm not sure if there's any reason to upgrade. The only thing I don't like is sometimes the Chromecast icon doesn't show up and I have to restart the app, but I doubt that's on the Chromecast end.
Overall, I'm pretty satisfied with what I have, but I've been looking at comparison websites and they aren't really telling me what I need to know, which is if I'm missing something good I shouldn't be missing.
Also, this is not really important, but I'm on Chrome and Google thinks "Chromecast" is a misspelling.
I find the dedicated remote the Google TV ones have quite handy, and it makes it a bit more of a tv-like experience when I'm binge watching stuff.
As for extra features, the remote has a microphone so you can access the Google assistant. They're also full Android devices so you can install apps directly onto them such as games and use the remote as a controller, though I'm not sure how many are supported since I haven't tried that yet. Perhaps more usefully if you have a suitable USB hub and webcam you can use it for video calls, or plug in a harddrive and watch videos from that. Other than that if you're happy with second generation one, I don't think you're really missing out on much.