OK here's my two-penneth, Having had both full & half covers for a RR that is used on a semi-regular basis ie about fortnightly.
1) +1 for Datatek's comment.
2) for long term storage use a full cover, but only a half on for my type of usage, or more regular.
3) the most convenient full covers are those that have a cord running in a hem around the base, so no tapes to pass under the vehicle & get messy from wet ground contact.
4) if your model has exposed rain gutters protect the front corners (top of the 'screen) to prevent chaffing.
5) I don't think any of them are fully waterproof (despite the manuf. claims) but even if they were the cover wouldn't be breathable, which is what you want.
6) the covers 'wick' in wet/damp weather & the resulting condensation will cause the cover to freeze to the roof during freezing overnight temperatures.
nb: The manuf. recommend pouring warm water onto the outside of the cover before attempting removal, but I've never tried it, also gale force winds & covers don't go together !
7) despite what the adverts say, cover manuf. do not make covers for specific models, they are generic so finding a decent fit is a question of luck. Usually the product can be returned for exchange, for example if a cover for a RRC is required don't 'click' on RR as it will be too big, a Freelander 1 or Jeep Grand Cherokee/Honda for example is about right.
Strangely (or not) enough I'm more inclined to cover the car during high summer in order to protect the (difficult to obtain) screen/window rubbers from UV rays
side by side cover.
Hope this advice is of help to you.