In general LPS and SPS are less tolerant of nutrients in the water (they like a little bit, but won’t tolerate the levels that a lot of softies will thrive in), require more stable parameters, and with stony corals you have to maintain calcium, alkalinity, and magnesium (along with other trace elements). An SPS dominant tank in general requires greater attention to water chemistry, higher lighting, and higher flow. LPS and mixed reef tanks are kind of in the middle, and soft coral tanks are the most forgiving in terms of chemistry, lighting, and flow. But the main fundamental difference between stony and soft corals is in the name, to form a stony skeleton, stony corals require stable and adequate alkalinity, calcium, and magnesium, soft corals don’t (they still use these elements to a certain extent, but they’re far less important).
One other thing to note, and one of the main reasons why people say SPS are more difficult (though not the only reason) is that maintaining the proper parameters is very different and the methods by which you achieve it are very different when you go from soft to stony corals. A large grown out SPS dominant reef can be very difficult to maintain alkalinity and calcium levels, whereas with a softie or even mixed reef, a simple two part will often keep levels stable.
