Well it all depends on the size of your RIB I suppose!
Keith could tow his behind his bike!
I prefer to use my Land Rover - of course.
I'm now onto my second Freelander - great at steep slipways and beech sites.
My first was a Diesel - my current is a V6 which I imported via Virgin Cars.com (Branson gets everywhere) and with the saving I made on the import had a Gas conversion done. With LPG at 35p/l its actualy cheaper to run than the diesel - and more fun to drive.
IMO The freebie is a great car for everyday driving - I do 100 miles a day round trip to work down the motorway - and also a very capeable off-road vehicle (The only issue being ground clearence - but you don't find many deap ruts on slipways and I wouldn't recoment towing your RIB over the ridgeway!)
Seriously though - one of the considerations to make is the weight combination of tow vehicle and trailer so that you have a safe and stable "outfit" when towing. Obviously if you have a big RIB then you need a big Tow Car - like a Defender.