Stylocoeniella is in the Astrocoeniidae family and Cyphastrea is in the Faviidae family. The Stylocoeniella typically has polyps that extend, similar to acros and other SPS, while the Cyphastrea has coralites and polyps more like a typical LPS.
No problem. I think I have a Stylocoeniella, but am not sure. It doesn't really match the pictures (smooth, where the pics of a similar species have nubs near the polyps, etc.). A good website for loose identification is whelk.aims. http://whelk.aims.gov.au/coralsearch/coralid_search.php
Check out Madracis sp. VERY similar to Stylocoeniella, but lacks the spiny developments around the polyps. This is given as a distinguishing characteristic in Julian Sprung's book Corals.
I was sold a gorgeous piece a while back as Madracis, but it has the spiny projections in the coenosteum, so I assume it is actually Stylo. I'd show a picture, but these are, of course, an sps and it's hard to capture this detail without a macro lens. Neat corals, nonetheless, and apparently quite rare in the hobby.