The PhotonS has dual rails and a different UV light matrix. The sliced files are .photons instead of .phonton and so some of the free software out there doesn't work on PhotonS files. So the S model as less support out there. Some say it is better to get a regular Photon and upgrade it to dual rails. I really don't know myself which model is better. Theoretically upgrading the UV light could get you shorter burn times and speed up a print. But they say that will shorten the life of the LCD screen. Then again at the low price it is worth it to find out. If mine died right now after less than a year I would still say it was worth it and just buy another. That and parts are available for these so you have a chance to fix them.