Matthew - there's quite a bit of experience of covers on the Forum. My advice would be to stay away from the heavier half covers and especially any with magnetic parts. They do mark your paint and especially so in wind and over time. Chesherman ruined his paint (black E85) with such a cover and I stopped using both of mine (I bought 2 different types to try and avoid the marking of paint). The best compromise I found was to go for a very light waterproof type (with elastic straps which went under the back and front of the car) - this still marks the paint over time and in wind but it only hazed the top clear coat and this could easily be polished out. The half covers keep the water from drowning your roof motor though so well worth using - or - we have a guy up here who will relocate your roof motor into your boot for £80.
At the moment I use a full cover from CoverZone called a 4 season Stormforce outside cover (about £150). I use two large fleece sheets, one across the front bonnet and wings, one across the rear boot and wings and then put the cover over the top. It has straps underneath the car which clip onto the cover and in high winds I tie (using soft rope) from one wheel across the front and rear of the M to keep the cover on and one soft rope just around the M at the bottom of the screen. I clean the M after every run and before putting this cover on. Bit of a farce but I've found it the only way to protect the paint finish.