Both of these cars were competing in the same class, but the difference is the solar array. All of the Nuna cars have had multijuction arrays made of III-V semiconductor cells. Such cells are very expensive, and are only used in satellite or concentrator applications, but they can have efficiencies up to 35%. Enough cells to cover a solar car can easily be more that a million US dollars. Sunswift IV, on the other hand, has an array composed of Sunpower C-60 solar cells, which have a theoretical efficiency of 22%. The record they set was for a solar car with silicon solar cells, and as someone mentioned, they did the run without a battery pack. The wikipedia quote for theoretical top speed is running solely off battery, which doesn't make it much of a SOLAR car.

Great idea, memorize two three digit numbers that can give you 6 places of pi. Now all you need is a calculator to divide them ;)