This kind of testing costs money to do, consequently, items that are UL listed tend to be a bit more expensive to cover those costs. The UL ones might have a different wiring insulation on them, or a different gasket material on the sealing box, or minor stuff like that so that they conform with the standards and CAN get that UL sticker on them. Then again, the UL ones might be exactly identical to the NON UL ones, just that they have not been 'formally' tested, so can't get the listing.
Quality wise, the UL listed product will be the same, if not slightly better than the non UL products. Both produce the same power, and have the same warranty on them. If you are doing some sort of setup requiring building inspectors though, then yep, you will have to spend the little extra cash to keep the inspectors happy.
BTW,I do own 6 of these panels, and love em.