Swift 1.2 to 2.0 conversion - guard vs if let

I am trying to convert some code from Swift 1.2 to 2.0. I have the below code in Swift 1.2

//enable OR disable keys.
if(discountAmountTextField.text.isEmpty){
    keypadView.disableNotRequiredKeys()
}else{
    keypadView.enableRequiredKeys()
}

There are two ways I can go about converting this to Swift 2.0 guard and if let

Here's how the code looks like with if let

//enable OR disable keys.
if let text = discountAmountTextField.text {
    if text.isEmpty {
        keypadView.disableNotRequiredKeys()
    } else {
        keypadView.enableRequiredKeys()
    }
} else {
    keypadView.enableRequiredKeys()
}

Here's how it looks using the guard syntax

//enable OR disable keys.
guard let text = discountAmountTextField.text else {
    keypadView.enableRequiredKeys()
    return;
}
if text.isEmpty {
    keypadView.disableNotRequiredKeys()
} else {
    keypadView.enableRequiredKeys()
}

I want to know what is considered as a more cleaner and appropriate way of writing. The guard looks cleaner to me but is there a rule of when I should be using one over the other? Is there any way to further simplify the method?

I'd use a where here:

if let text = discountAmountTextField.text where text.isEmpty {
    keypadView.disableNotRequiredKeys()
} else {
    keypadView.enableRequiredKeys()
}

It combines two of your failure cases into one.