SM feels too dated at the moment. There is no WASD functuality, and the overall viewing system takes some time to move around. (the lack of a fly-by view for those who want to adjust their position in a seat, for example) Some of the editing is a bit odd too, and most of the trains are completely untextured. It's options of coaster types and moddability are what makes up for that and makes it worth buying, particuarly if you want to work with rides that NL doesn't have. Plus, there's muitiple coasters per scene, though hardly anyone ever uses that feature.
Back to graphics, in NL, stations, lifts, and catwalks can be replaced via 3ds. In SM, you can't simply map textures onto the train, nor somehow get reflections to work. The process of importing models can get to be very annoying as well, since the object disappears from your track/park if the file is changed in anyway.
Oh, and plus, NL has dozens of 3rd party tools. SM does come closer to WYSIWYG though, since the preview functuality activates faster in most cases.