When to use Xamarin.Forms vs Xamarin Native?

Xamarin.Forms

Pros

• Create one UI for all platforms
• Use basic components that are available on all platforms (like Buttons, Textfields, Spinners etc.)
• No need to learn all the native UI frameworks
• Fast cross platform development process
• Custom native renderers give you the ability to adjust the appearance and feeling of controls

Cons

• It's still a new framework and still contains bugs
• Especially Windows RT is not yet stable
• It's sometimes slower than accessing the native controls directly
• Custom native renderers have boundaries and are poorly documented

Xamarin.Android, Xamarin.iOS, Windows Phone, Windows RT

Pros

• Create one UI per platform
• Easy ability to adjust the platform specific UI features and components
• Get the maximum out of your UI

Cons

• Cross platform development process is much slower
• You need to learn all native UI frameworks
• You may result in having duplicate code (as you describe almost the same UI in three different platform specific ways)

In addition to that Xamarin says on https://xamarin.com/forms:

Which Xamarin approach is best for your app?

Xamarin.Forms is best for:

• Data entry apps
• Prototypes and proofs-of-concept
• Apps that require little platform-specific functionality
• Apps where code sharing is more important than custom UI

Xamarin.iOS & Xamarin.Android are best for:

• Apps that require specialized interactions
• Apps with highly polished design
• Apps that use many platform-specific APIs
• Apps where custom UI is more important than code sharing

Aside from the points shown in the Xamarin Website, you should also consider:

Your team's background

Does anyone in your team have a previous experience coding natively for Android or iOS (using Java or Obj-C/Swift)? This experience is reused in native Xamarin, but they'll have to learn another platform to code in Forms.

The same is true if they know nothing about iOS/Android but know some XAML. Sure Xamarin.Forms are different from WPF/Silverlight, but knowing XAML surely helped me using Forms.

Performance

Xamarin Forms still has some issues, such as initialization time. You should consider whether or not this will be a problem for your final user. I wouldn't bother to wait a while for a dashboard app that I use once in a while to open, but I'd be angry if the same happened to a messaging app.

Time until delivery

Since the code sharing is bigger when using Forms, you may expect a quicker delivery time.

Complexity of your app

Considering the previous point, using Forms you may stumble on some show stopper bug (it's a very new technology after all) that will null the time gains. Consider the complexity of your application before choosing.

From the Xamarin Website

Xamarin.Forms is best for:

• Data entry apps
• Prototypes and proofs-of-concept
• Apps that require little platform-specific functionality
• Apps where code sharing is more important than custom UI

Xamarin.iOS & Xamarin.Android are best for:

• Apps that require specialized interactions

• Apps with highly polished design

• Apps that use many platform-specific APIs

• Apps where custom UI is more important than code sharing