I own 3 Rokus that are about 4-5 years old, and I'm happy to share my thoughts. For the 2 places that I could easily get a wired ethernet connection to, I went with the Ultra mainly for that reason. I assumed that a wired connection would be more reliable than Wifi - not sure if that's true, but I've never had any issues. I have a much cheaper Roku for upstairs that uses Wifi. It's a little slower to respond to button presses, but it's not a dealbreaker. I think the price difference back then was similar to now, about $100 to $30.
Another thing about the Ultra I liked was that the remote has a headphone jack directly on it. When my kids were really young, I used that feature occasionally when watching movies at night, but I haven't done that in a long time. A quick search shows me they eliminated that feature, but it looks like there's a direct bluetooth headphone mode now. I don't think the cheaper ones have that.
I'd say if you're planning to use Wifi, don't need the Roku to connect directly to headphones, and don't mind a very slight "lag" with the input, you'll probably be happy with the base models. Again, this is all based on older models, so the lag might not even be an issue anymore.
