As I currently understand it, which Start experience you receive in Windows 10 will depend on a number of factors. Those with traditional PCs or 2-in-1s will get the Start menu by default. Those with tablets will get the Start screen by default. And those who upgrade should receive whichever Start experience they were previously using. But it doesn't really matter, since you can switch between the Start menu and the Start screen at any time.

Well, almost any time. As with some other desktop configuration changes—like display scaling—this change requires you to sign out and sign back in before the change takes effect.

To switch between the Start menu and the Start screen, navigate to the desktop (if required), right-click on a blank area of the taskbar and choose Properties from the pop-up menu that appears. The Taskbar and Start Menu Properties window appears.

Click the Start Menu tab.

Now, check (or uncheck) the option titled "Use the Start menu instead of the Start screen." Then, click OK. You will be warned that you need to sign out before the change will occur.

Here's the normal Start menu:

And the Start screen after making the change:

Note: This option does not impact how Windows boots up. So if Windows was previously configured to boot to the desktop and you enabled the Start screen, for example, Windows will still boot to the desktop.