Both Jesus of Nazareth and Siddhartha Gautama (Buddha) are certainly historical figures.  I am not sure what is to be gained by asking which of two certainly historical persons is more certain. How much more certain can we be than certain? ,But if one had to do the exercise, then the historicity of Jesus is considerably more solid than that of the Buddah.  There are no contemporary accounts of the life of Gautama.  The first written records for Buddha come from Ashoka, the leader of Northern and Central India, in 249 BC.  This is nearly two hundred years after he died.  By contrast, there are well over a dozen authors who mention Jesus from within one hundred years of his death (Josephus, Tacitus, Matthew, Mark, Luke, John, Peter, Jude, James, Paul, Clement, Pliny, Ignatius, Polycarp, author of Didache, author of Hebrews, Suetonius, and several others).  The one who mentioned Buddha in 248 BC was a believer.  I do not know the earliest non-believer to mention Buddha, but it is clearly later than that.  A number of those who mentioned Jesus within one hundred (not over two hundred!) years are non-believers.  By any conceivable measure, the historicity of Jesus is much greater than that of the Buddha.  However, like I said, it is certain that both religious leaders are real persons who actually lived.  One cannot explain the existence of the religion Buddhism if the person known as Buddha did not exist.  This is even more true of Christianity and Christ.