Hello Fatboy,
There are two bits of good sense which I think mighyt apply here. The first is It's always advisable to keep any trailers weight as small as possible, adn the second is that don't end load tarilers as this reduces the outfits control of instability, so for these two reasons alone the roofrack option makes sence.
There could be some other considerations, for example not all caravans have the necessary wall strength to support a bike rack. and it would add some length to the caravan that might compromise close quarters maneuvering.
If you have a roof rack, then you can take the bikes out when not using the caravan - more versatility.
There are some downsides to roofracks, the first is can you reach to put them up there?, and when traveling around away from the caravan site you will encounter more wind noise and loose some MPG.
Take you choice.