Both are amazing sensors, and I happen to own both from ZWO: a cooled 533MC and a 585MC (non-cooled).
I have to say, the 533MC gets you a lot of the way there with planetary. It has a decent frame rate and plenty of coverage.
But the 585 is a planetary BEAST.
A couple of things to consider:
Sensor size.This becomes largely irrelevant, as you will end up cropping down to 640 X 480 (or even smaller) to both reduce frame size, and achieve even higher frame rates.
Bigger sensor can be useful in finding planets though a Barlow, but once found and centred you will want to crop in anyway.
Over cropped in, there is plenty of leeway for region of interest. Just make sure your polar alignment is spot on.
Frame rate.The quoted frame rates on ZWOs site are the highest possible at maximum resolution. Shooting planetary at max resolution is a huge waste - as even Jupiter will only cover a fraction of the sensor.
With my 585MC I have achieved some ridiculous frame rates of 150FPS + which is great for beating the seeing.
I've gotten close with the 533, but 585 is simply faster. I think even higher frame rates are possible with quicker SSD cards and the like - I have been capturing on a Mele Quieter 3Q.
Pixel size. Both sensors have what are considered small pixels. For planetary imaging, oversampling is the norm, and the 585MC has the edge here again. The smaller pixels lend towards oversampling and the arguable advantages of such a method of capture.
Full well and bit depth.(From memory) The 533 has the advantage here. However, both of these things aren't really that important for planetary, as you will generally want to shoot in 8 bit mode anyway ( for faster frame rates - frame rate is KING) and at higher gain (I've shot up to about 450 gain with the 585MC and it still gives amazing results).
Food deep sky - invert everything I just said.
Near IR sensitivity.The 585MC wins out here again. It has greater sensitivity in the near infrared. What's the advantage there? Well, to be honest, using and IR pass filter, I have managed to eek out some additional minor detail, but never been able to successfully combine those IR images with colour captures to gain greater detail.
But the capability is there with the 585MC.
In summary.From a purely planetary/solar/lunar perspective - I think you have to give it to the 585MC. If you already have the 533 - then you could get 85% of the way there - maybe more.
If I had two comparison images - I would show them here, but in every instance I want to shoot some planetary, I've used the 585 as the capture camera - and used the 533 on a guide scope to help me centre targets
I've certainly shot planets with the 533, but i've always swapped it out for the 585 if ever the seeing got good, or a was having a serious crack at a planet.
For reference:
Image 1: 585MC With Skywatcher 127 Maksutov
Image 2: Skywatcher 127 Mak with the 585mc again
Image 3: 585mc through an svbony 80ED
These images are through tiny scopes! The power of these cameras is amazing.
I wish you luck with whichever camera you get.
The new cooled version of the 585MC looks like one of the greatest jack-of-all-trades cameras you can get today.
