When should I use createSignal() vs signal()?

I see that createSignal() returns a SignalGetter<T>, while signal() returns a WritableSignal<T>.

What are the key differences between these two in Angular, and in what scenarios would I use one over the other?

The createSignal() is used internally when signal() is defined. I do not think you will need createSignal() since it seems to be an internal angular method and not even documented as a public API.

The code snippet is as below.

signal.ts#L79

/**
 * Create a `Signal` that can be set or updated directly.
 */
export function signal<T>(initialValue: T, options?: CreateSignalOptions<T>): WritableSignal<T> {
  const signalFn = createSignal(initialValue) as SignalGetter<T> & WritableSignal<T>;

So the signal is a function that returns the value inside, which is created by createSignal.

Then we also add the additional types using & where it is a union of SignalGetter (Signal reads) and WritableSignal (Signal Writes).

Then we add the methods for updating the signal (WritableSignal part of the signal).

  signalFn.set = (newValue: T) => signalSetFn(node, newValue);
  signalFn.update = (updateFn: (value: T) => T) => signalUpdateFn(node, updateFn);
  signalFn.asReadonly = signalAsReadonlyFn.bind(signalFn as any) as () => Signal<T>;

  if (ngDevMode) {
    signalFn.toString = () => `[Signal: ${signalFn()}]`;
    node.debugName = options?.debugName;
  }

  return signalFn as WritableSignal<T>;