If you have a Plus 4 with the 2 litre Duratec engine the larger storm cover is a the better option as it will keep water out of the engine compartment.
The 2 litre Duratec does not have a cover over the spark plugs, and are therefore unprotected from driving rain water.

Ian
2011 Plus 4
Wild Mogs Reims 2019, Goodwood 2018, Isle of Man 2017, Carried on to Caramulo" 2016