The differences are:
- The Roadster has plusher seats that are generally considered more comfortable.
- The Roadster has roll-up windows. With the exceptions of Intermeccanicas, Speedsters have side curtains like the originals.
- The Roadsters have a taller windshield and top.
Most folks consider the Speedster a little cooler looking, but the Roadster is a bit more comfortable. However, at the moment Intermeccanica is the only Roadster manufacturer. They're wonderful cars, but a bit expensive. If price is not a problem, they sell both models.
I've driven my Speedster on two trips with over 1000 miles each, and was surprisingly comfortable - of course I had low expectations ;-). Getting out every couple of hours to fill up worked wonders.