At this point Xamarin.Forms is a mature framework while MAUI is a blog post. That makes them hard to be objectively compared.

In general the idea for MAUI is to be a further evolution of Xamarin.Forms. Personally I don't see anything revolutionary (and even important) announced. The main difference will be that MAUI will be a single project with multiple outputs instead of multiple projects for each platform. There should be also some improvements in managing common assets as it was usual to place assets in platform projects and not in the common project, but most of that is possible in Xamarin even now so it is hard to say whether it is some kind of improvement or just a marketing trick.

Also support for macOS should go from beta to production. Again it is hard to say whether it will be some significant boost of fairly low quality in beta or just a marketing trick.

MAUI is the next generation of Xamarin Forms with broader platform support. The first preview will be available in .NET 5 in Nov 2020, and the first production release will come with .NET 6 in Nov 2021.

Xamarin Forms will be supported for one year after MAUI is introduced in 2021, then will be deprecated in favor of MAUI.