If you mean a modern caravan its simply that they are bigger, ie if you want the biggest possible caravan it will be a twin axle. Twin axle 'vans of similar size to single axles were made in the 80s but pointless buying one as you have the extra expense of buying 4 tyres which you would probably need to do if buying an old one off ebay for example.
Our twin axle 'van is excellent to tow and extremely spacious and comfortable when pitched. You do need a motor mover to manoeuvre it into some tight spots though.
Hi Paul, It basically depends on what towcar you are using. If you have a suitable, large towcar possibly a 4X4 I would easily recommend a twin axle but as always check the weight of the fully loaded caravan first.We have had many caravans, mostly twin axles but two years ago bought a Single axle Bessacarr Cameo 495 and it towed fine.Changed it recently for another twin axle Bessacarr 645 as we liked the Island bed layout.(We checked the Island bed layout on a few Single axles but thought they were a bit tight on space). Agree with Al& Mel regarding movers, good idea.Arguably, the dual axle possibly tow a little more stable on dual carraigeways and motorways, but again this depends on the towcar....