Basically, the only difference is that the turgo has the jet enter about 20 degrees angle to the plane of rotation. This in turn makes it possible to use a larger jet relative to the turbine pitch diameter, therefore for a given head and target rpm the turgo can handle more water and produce more power. In theory the out of plane jet angle reduces the total turning angle slightly, with a slight reduction in efficiency. In practice, at least for smaller mass produced units, the bucket profile and material make more difference than the jet entrance angle.
One of my customers sells systems based on Peltons and also based on my turgos. He has a chart comparing efficiency of the two. For very small nozzles the Pelton and turgo are comparable, for larger nozzles, the turgo has an advantage.
Since in practice there is not any efficiency loss, and in fact a modest gain with the turgo. The reason we don’t make Pelton’s is that for any site where a Pelton is applicable, a turgo will work as well, while there are many sites where a Pelton would either limit the flow or require a larger, slower, more costly turbine.