Below or Above the Spanish Steps in Rome

I honestly don't think it matters. The Spanish steps aren't that interesting, there's too many people on them to even see the steps.

I'd not stay either side.

I prefer Prati, or near Campo de' Fiori. My favourite now is Parioli, but that's different.

Because the Spanish Steps lead up to a park, i.e. Villa Borghese, the answer to the question isn't that useful. There are several luxury hotels near that corner of the park (and, in that sense, "above the Spanish Steps".) But Rome is a big city, and there are many desirable hotel locations. It's kind of like asking, when I visit NYC, should I stay in the theater district or in boutiquey SoHo? It depends on what you want in a hotel, and perhaps on where you want to spend the most of your days. It doesn't reduce to a single question like the OP.

Like any big city, getting the price you want for the days you want is a huge factor. You need a really good reason to consider only one location for your hotel.