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+1 agree with Steffen.
Comparing my 7" f/15 Mak with 8-12" newtonians:
1. Contrast in the Newtonians depends on the planet, and also the size and configuration of the diagonal spider. A traditional Newtonian with 4-vane spider will show horrid spikes that degrade the image of Mars quite badly, yet the view on Jupiter / Saturn isn't so bad.
Operating at the same magnification the 7" f15 Mak out-resolves all of the 8" newtonians I've tried it against. A 12" f5 is a tad better - just - but there's the shorter focal length to contend with, which means the eyepiece selection is fundamentally different.
A long Newtonian, say f7 or f8 with a single curved vane spider, or alternatively a maksutov Newtonian (no vanes at all) should be considerably better. By way of example the 16" at Mt Bowen (f7) provides superlative views of the planets and easily better than a C14.
The best - bar none - is a schiefspiegler (unobstructed off-axis reflector, for the uninitiated). But at f23 they're not much use for anything else.
2. The larger catadioptric scopes (like the 14" Celestron) have a very long focal length - so long that it only provides high magnification, or extreme. At the images scale of these scopes poor seeing will badly degrade image quality and it's true that you may well see more by stepping down to a smaller scope around 8".
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