Darren, don't pay too much attention to the external dimensions of the SIB, its the internal floor space that matters. You may notice that some have pointed rear tubes that extend quite a way back past the transom. Some like zodiac have rounded tubes at the rear. This can add valuable floor space. My 3.4 zodiac has 8ft of internal space and quite wide compared to many others of a similar size.
Oh and the internal stoage 'pot' is more of a pain that being useful. It has to be inserted before the front tube is inflated or you can damage the tubes.
Other things I would look for. How many seats can it accomodate? How easy is it to get spares or repairs? Does it have space to fit transducer to the transom? where could I fit GPS, fishfinder and rod holder/s?