-quite aggressive and over the top marketing.
Everything Black Magic Design sells is: "...the most advanced in the world...", "...the industry’s most powerful..." and "...the greatest in the world..." etc.
It's been a few years since I used Fusion so I'm not quite up to date on that, but I use Nuke almost every day.
Both are powerful node based compositors. Fusion have perhaps a bit wider tool set, for-instance more/better particles, 3D rendering and 3D text. Nuke focuses on comp and can handle hugely complex comps. It has very good keying and image manipulation tools, and you can have control of almost everything. -and the inbuilt 3D camera tracker in Nuke is quite good and handy. -but the particles is a pain, and 3D work is primitive.