What Size Thickness Wetsuit Boots Should I Get For UK Waters?

I live on the Isle of Wight, UK, and I cant decide whether to get 5 or 7mm Oneill Heat Boots! I've been told that 5mm should be fine and keep me warm and that the 7mm will be a nightmare to put on and take off! Are they THAT hard to put on or is it worth the extra warmth?