[Robh (Rob)]

For the sake of comparison, let us assume you are using a 30mm eyepiece with 80 degrees apparent field of view.

In a 12 inch (300mm) f/6 scope, primary mirror focal length = 300x6=1800mm, the magnification would be 1800/30=60.
True field of view = 80/60 = 1.33 degrees.
At f/6 the scope length is about 1.8 metres. For most, this would be too high to view through when the scope is pointed upwards and observers will need to stand on a box. A 12 inch, f/5 scope, is a better proposition.

In a 12 inch (300mm) f/12 scope, primary mirror focal length = 300x12=3600mm, the magnification would be 3600/30=120.
True field of view = 80/120 = 0.67 degrees.
Note that the scope length would be around 3.6 metres. You would need a ladder to get to the eyepiece.

Visually, the f/12 has half the true field of view but twice the power. With the smaller field you would have less issues with coma but on the other hand, the scope is far more cumbersome to handle.

If you are looking at Schmidt-Cassegrain designs, the 12 inch scopes are far more compact but have their own inherent problems with coma and field curvature.

Regards, Rob.