at the present time you can use both, and some people do. You can do more with custom code, Geopresence, and security devices including locks in the classic app.
You have to use the new app to set up hubs released in 2018 or later. You also need to use STSC (the new app) for Samsung brand smart appliances and televisions. There are also some new device integrations which only work with the new app.
Other than that, it’s up to you.
See the following community FAQ (the topic title is a clickable link)