Have the MINI OEM car cover and it works a treat
Have used covers with a few other cars and have found the MINI one to pretty damn good by comparison ... it comes with an integrated storage bag and fits like a glove ... ventilation is fine
I have Sat Nav so I have never needed to use the hole for the aerial ... from past experience I would strongly recommend detaching your aerial if you have the standard one, leaving a gap in the cover for an aerial will ALWAYS let water in and because the roof is flat it NEVER drys properly ... this is not a problem that is unique to the MINI but a general problem with car covers
For thefew extra dollars I would strongly recommend the OEM cover