First, it is difficult to exploit apertures over about 10" to 12" for planetary work unless your seeing is routinely pretty good. I do a lot of planetary observing in my C14, but it is rare for me to work at close to the full potential of the scope. When I can, I see clear detail on Ganymede, but that is a couple of times a year.
But to your question.
The problem with the scopes you mention is that if they are made to f/20, they get very large and very difficult to mount.
The 14.5" Parallex DK is 56 inchs long and weighs 85 Lbs.
The secondary obstruction is a pretty large 25%.
The mirror is made by Royce (one of the best).
The scope will have about 275mm of clear aperture.
It will cost about $13,000 and you will spend another $7K on a mount for it.
For about $4000, you could buy an Orion Go-To dob and have someone refigure the mirror and put in an optimized size (18%) secondary mirror.
If you were serious about planets, you could do as well with a 12" Go-To dob
This would give you motorized tracking, and would negate the need for a 6 foot high saddle for your 85 LB cass.
And if you plot the MTF curves for the two scopes, you will quiclky see that the contrast performace of the 12" scope with 18% obstruction will be equal to the 25% obstructed 14" Cass.
Contrast transfer is everything for a planetary scope, and the 12" optimized Newt would be the winner here.
It costs less, is lighter, provides a better viewing position, and provides better contrast transfer.
A small obstruction Newtonian gives performance that is difficult to distinguish from an equal sized APO. The APO owners might disagree, but having owned both a 6" 18% obstructed reflector and a 6" APO, I would call the difference in performnce to be within a cat's wisker.
To me, ultimate planetary scope is 12" Go-To Newt with 18% obstruction. You still come up against seeing though, but you will get a better planetary scope out of a custom 12" Newt.
If you were really serious about this, you would do an MTF plot and see quickly that the Newt is on par at a fraction of the price.
I strogly (but politely) disagree!
14.5" with 25% telescope will be MUCH better than 12" with 18% c.o.
1. The contrast will be equal ONLY at middle frequencies. At higher frequencies, where details are the most numerous, the larger scope will have higher contrast.
2. larger scope will have higher resolution and will show significantly more small details.
3. Larger telescope will show brighter image at the same magnification and in such will win in contrast and colors rendition.
4. Photon noise will be lower in larger telescope - it will win in CCD welcoming.
I am wondered, that so experienced amateur astronomer can write such a nonsense, as you did, Eddie.