Best board to learn and progress on?

All the difference in the world.

I heard somewhere (may not be scientific fact, but the point holds)
that every cm in width is equal to 8cm in length.

It sure feels like it!! Yeah, the extra width makes a huge difference in ability to plane and get going in the light stuff. Thing is, the SP doesn't feel big at all, especially in the air. And once it fills in, you don't have to hop off, if you don't want to. It's still fun in 20, not unruly.
I hope all the new Phattie models are as good as Aggression's.

Maybe Wolfie or Kuds or someone who demoed the SP will hop on here and comment. Or A-bomb about his Exile.