Dig Into iOS 12 Beta Feedback
If you're using an older version of iOS or an older iPhone/iPad, it might be a good idea to hold off on the download until you've dug into some of the early feedback from iOS 12 beta users.
Older devices typically have the most trouble moving to new software. Despite the performance improvements on board, devices like the iPhone 5s and iPad mini 2 could struggle on iOS 12 beta.
If your device is currently running an older version of iOS and that version of iOS is performing well, you'll want to be absolutely sure about your move to the beta.
There's no way to go back to anything older than iOS 11.4.1 once you make the move to iOS 12 beta. Apple's closed off iOS 11.3 and older versions of iOS and it won't open them up again. This is why you need to be sure of your plans.
This is why we recommend digging into early feedback from people using iOS 12 beta on your device. This feedback will help you learn about potential benefits and problems.
If you're unsure about moving your device to iOS 12 beta or a newer version of iOS 11, do yourself a favor and dig through opinions of beta testers.