As someone who owns Forza 6 on Xbone, and and now Forza 6 Apex, I can confidently say that Apex looks MUCH better than the original game on Xbone.
Don't get me wrong, Forza 6 is one of the best-looking console games I've ever seen, but you can see where they cut corners with the graphics. Cutscenes run at a reduced framerate, reflections run at a reduced framerate, aggressive LOD reduces lots of detail at distance, and no AA makes the game suffer from jaggies.
The PC version solves all of these problems (but you have to go into the video settings to max everything out). It looks incredible. Literally everything runs at a butter smooth 60fps, 4xMSAA eliminates the jaggies, and everything just looks more detailed in general.
It's definitely not a full game; it only has a fraction of the content that the Xbox One version has. I think it's more of a tech demo to show what Forza can look like on a proper gaming PC.
If you use Windows 10, there's no reason not to try it out. It's free after all.