Caravan Air Conditioning - any ideas please?

Oldgirl and Staffy

We are thinking of buying air conditioning for our Hymer caravan but are totally confused as to whether it would be better to buy a portable unit or pay more and have a fixed unit on the roof.   

The roof units seem very heavy at around 42 kilos.  We'd love to hear from anyone who has gone into the issue in detail and can offer some advice based on their experiences