There is no difference as to the actual Outlook Pre software- its the same if you have the toggle or not. If at least one account in the Outlook profile isn't supported in the new outlook, the toggle shouldn't be shown.
Getting started with the new Outlook for Windows - Microsoft Support
If you get the new Outlook from the windows store, it wont have the toggle.
It's possible to hide the toggle using a registry key -
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General
DWORD: HideNewOutlookToggle
Value: 1
If you don't have the choice of the good Outlook when you search for outlook (as seen in my screenshot), the good Outlook is here -
C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXE
or
C:\Program Files (x86)\Microsoft Office\root\Office16\OUTLOOK.EXE
Open it and pin to start menu and taskbar.