Hi Jay, I've always used a half cover on both my Zeds as I keep them outside (garage full).
The first one I bought came from Roadster Solutions and cost about £50. It was light and made of a silver waterproof material using elastic straps which went under the rear and front to hold in place (with rear door mirror pockets). It worked well and kept rain out the roof motor - I used it every night.
Then a group purchase of a 'Buzz Top' at £55 or thereabouts came up on the forum. I fancied a heavier cover and this was made of a much thicker and heavier material. It was held on by Velcro straps under the door mirrors and magnets along the edges set into the material seams. I used it for months - the material changed colour from black to light green due to light but it also marked the clear gel coat of my paint - clouding it along the top of the doors and the below the rear window - it was a pain to buff out.
So I saw a more expensive 'UKCustomCovers' cover for sale on the Forum which I thought might be better made and went ahead and bought it.
It was far better made and had what seemed a better seam as well as being secured through being trapped in the boot - it was an excellently made cover and came in it's own case - quite superb. However it was the worst for marking my paint causing scratches and marks which called for work to correct - I stopped using it within a week.

(Both heavy covers are in my garage and will not be used again nor sold on) My mate is in email with this company asking for his money back as the cover he purchased ruined his black paint with deep scratches and marks - he was heart-broken
I have reverted to my initial light material silver cover from Roadster Solutions which of all three does keep the rain and muck off the roof - the only issue with this one is that the elastic straps do mark the gel coat if it's windy - but this is easily buffed out with some paint corrector from AutoGlym.
I guess the answer is keep your Zed very clean if using a cover as they all mark the paint but the best one for me was Roadster Solution which is easy to store in the boot (it has it's own zip up case) and very light but effective. If you are OCD over your paint then don't get a cover but keep your drains clear (not a hard job once you've done it once).