Whats the difference in 9.3.5 and 11 ?

Too numerous to mention here. Here is what iOS 10 offered over iOS 9:

Download iOS 10.0 - iOS 10.3.3 Information

and here is what iOS 11 offers over iOS 10:

About iOS 11 Updates - Apple Support

to give you an idea.

The biggest difference is all 64-bit and all 64-bit coded app support.

Any older 32-bit apps that haven’t been updated to 64-bit code will NO LONGER FUNCTION under iOS 11.

You will, also, no longer be able to access that data in the older 32-bit app any longer under iOS 11.

See my posting here.

iOS 11: WHAT TO KNOW BEFORE YOU UPGRADE!!!

New features in iOS 11 - Apple Support