i have the LT268 (which is an open mould), so i think tantan and others make the same frame, and i'm very happy w it. If i were to buy a frame today though, i'd go w the speeder 55. It's newer, lighter, it looks very nice, and i'd be very curious to see how it rides.
That being said, i dont think you can go wrong with either. The 268 is substantially cheaper. If you keep it several years, it shouldn't matter too much, because speeder is still great value, but there is a difference.