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Hi
If we take a step back for a moment, you want a scope that is great for the planets that will give better views than your Mak, which should give you good views anyway and teamed with an 2X barlow gives 5400mm (F30) focal length, which is more than enough. The seeing wont allow you to use longer focal lengths / magnification very often anyway.
To step up in aperture, resolution / mag. for visual and maybe basic webcam style imaging, the best value to me would be an 12" - 16" GoTo Dobsonian An 12" F5 and an 2.5 Powermate will give 3750mm ( F12.5 ) which I would suggest is an good focal length / ratio to view or image the planets. If you have good seeing quite often than you can be using 400 / 500X magnification, presuming collimation is good. That mag. will show you plenty of detail on Jupiter, Saturn and Mars, dust storms allowing.
The 16" F4.5 Dob gives 1800mm F/L, with 2.5X Powermate - 4500mm,
which is a good combination but the scope is just that bit heavier, bulkier to move around and that aperture needs great seeing most of the time if you want to use higher magnifications.
Suppose it comes back to what your budget is and where the scope will be set up, permanent or mobile.
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