John is quite right, diesel ribs are the business, but before taking the plunge you need to carefully consider what you intend using the rib for.
Over the years I have worked my way up to an 8.1 metre Scorpion with a 250 hp Yanmar diesel. This is a perfect cruising rib which lets me travel for less than a quarter of the fuel cost than I incurred with my previous boat - a 7.5 metre Scorpion with 175hp outboard.
There is a down side in so far as it is not quite as much fun playing in surf, and my access to some of the tighter coves around our coast is now restricted. The whole rig is heavier so if your towing there are trailer and vehicle aspects to consider also.
However the thrill of only having to pay for red diesel, for me, far outweighs anything else !