I use a Dahon Impulse 7 with Primo Comets. I commute on it every day and do journeys of between 12-60km and I can tell you the Comets make all the difference in the world. They are also very, very puncture resistant (important if you intend to commute) which alone puts them in a different league to most 20" tyres. If your going to be using the bike on a regular basis Id recommend the Impulse 7 beacuse its well made, resonably nippy and at that price you can afford to have it stolen. Get the Comets fitted at purchase, get a dscount because they cost a fortune and spend the money you saved over a fancier bike on flapjacks and banana's.
Oh and get a better seat!