When should I use optional channing and when should I use the 'if let' construct?

Optional chaining is basically the same as optional binding. Optional binding allows you to unwrap a single value, whereas optional chaining allows you to unwrap a property "within" an object. Let's say we have a class called Car. This car might have extras, but not if it is the "cheap model". Moreover, one of the extras might be airConditioning, but that is not guaranteed. This is how that scenario would look like:

class Car {
    var extras: Extras? // Optional, so possibly nil
    ...
}

class Extras {
   var airConditioning: AirConditioning? // Also optional, possibly nil
   ...
}

class AirConditioning {
   var manufacturer: String
   ...
}

// Let's create a beetle. Extras and Airconditioning might be created afterwards
let beetle = Car()

Let's say we want to get the manufacturer of the airConditioning extra of the Beetle. As both extras and airConditioning are optional, we have to make sure to check it is not nil and unwrap everything one after another:

if let extras = beetle.extras {
   if let airConditioning = extras.airConditioning {
      let manufacturer = airConditioning.manufacturer
      // Now we can use manufacturer
   }
}

But we could also access manufacturer "directly", using optional chaining:

if let manufacturer = beetle.extras?.airConditioning?.manufacturer {
   // Now we can use manufacturer
}

This code only makes it to the body of the if-statement if extras is not nil and finally airConditioning is not nil.