The Excalibur Mamey produces a fruit of extreme quality. It is longer and not as ovoid as a typical mamey (Pace, Pantin, etc.) with more scaling on the surface. The flesh is very sweet, a dark reddish brick to burgundy color, very moist, and completely fiberless. The tree itself has no differing growth characteristic (not dwarf or any more compact) and is not particularly precocious but I was told the fruit well makes up for it. On the other hand, the Lorito is more precocious but the fruit itself is of not of the same quality level. From what Richard said, and he has both varieties to sell, the Excalibur is the hands down choice. The origin is thought to be from Mexico.
Let me know if you have any other questions.