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The F number of a telescope takes into account the aperture. So to get an image of the same brightness you only need to compare the F number not the aperture.
This is where it gets a little more complex though, if an object has a brightness of 1 mag per arc second. The lower focal length scope (the refactor) would have a scale of 1.238 arc seconds per pixel (kaf8300) and the reflector would have a scale of 0.928 arc seconds per pixel.
because the refractors pixels will see more of the sky than the reflectors, and objects have a fixed magnitude per arc second you can't directly compare the images from the two scopes.
The reflector also has the secondary mirror that obstructs the primary, which is not taken into account when they quote an F number.
What you need to compare when looking for a new scope is the focal length, especially as a beginner as this will impact dramatically on the way your images look (far more than an F number).
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