Before I begin, let me just say that both materials have their pros and cons, and I use them both in my own clinical theater. I use Emax CAD almost exclusively for full coverage restorations, and Lava Ultimate is gaining ground for me in the partial coverage department.
If we just look at purely esthetics of the materials, it is hard to beat a glazed ceramic. It adds tremendous depth and brilliance to the restoration that rivals enamel and dentin. Composite resin materials can be polished very well, but rarely achieve the “glass-like” appearance of a fired glaze surface. Plus, the larger the restoration, the more a polished composite resin loses this vitality. Ceramic, on the other hand, gains vitality and brilliance as it covers more surfaces.
So, if a simple onlay is being done, a milled composite resin can easily blend into the dentition because it is part of the overall appearance of the tooth. However, a full coverage restoration will most likely blend in better with a ceramic that has an appropriately glazed surface.
This was a case where a Lava Ultimate HTA2 crown was compared with an Emax CAD HTA2 crown. The Emax CAD crown was selected and bonded with Multilink (yellow.)