Currently, there are two different versions of the web UI: the original one, now called the "Legacy" UI, and the new one.
Google is gradually adding features and settings from the old UI, to the new UI. They're not close to being finished. There is a small team of software developers working on it, and since tens of millions of users are involved, things tend to get done in a slow, cautious process, to limit consequences of potential mistakes.
The old UI will not be removed until the new UI is at "feature parity", meaning, it has all of the currently-supported features and settings found in the legacy UI. Billing features were recently added to the new UI, for example, and voicemail management settings will be added soon.
So: use whichever UI meets your needs; settings made in one will be reflected in the other. You may eventually find that you will be automatically redirected to the new UI, if the setting or feature is supported there.