Reference clock of 400 MHz oscillator or 20 MHz and multiply by 20?
My Question - is it better to use a crystal oscillator that provides 400 MHz or use a lower frequency such as 20 MHz and then multiply it?

It depends what you mean by 'better', and what you're going to use this synthesiser for.
Both routes have advantages, and what's important to you will dictate which advantages are more important to you.
Using the multiplication route has the advantage of cheaper components and more flexibility. The fact that you're asking the question probably means that this is the way for you to go.
If you want the best phase noise and spurious performance, say you're designing a high performance professional piece of equipment for sale, then you'd use the external high frequency clock source. If that was one of your considerations, you would probably know up front.