Whatever it actually is, the statistics have historically been pretty grim. The genetics of schizophrenia must be like a lottery. It may be spectrum disorder of several related conditions, or it may be a single entity. I think the considerable variation between individuals can be explained by different stages or different degrees of severity of one disease but many psychiatrists believe it could be four or five different illnesses that look alike. A lot of complex brain chemistry is involved in understanding what is happening in schizophrenia and even though the brain imaging technologies available within the last ten years are providing a wealth of information, the brain is still basically an uncharted universe.
Schizophrenia is generally more disabling.
Quite a few people are diagnosed with schizo affective disorder which has some symptoms of both, and it is almost fair to think of schizophrenia and bipolar disorder as being two ends of a continumn. They are genetically related as well, and you could have a grandfather who developed bipolar and you might develop schizophrenia. Descriptions of conditions like schizophrenia started to appear in Europe around 1850 and Kraepelin in 1878 followed by Beuler in 1908 described it quite well. He must have thought there was more then one kind. It has probably been around for a much longer time though, because it has a fairly even distribution around the world.