What is the difference between 8T gears "Type 1" (3647) and "Type 2" (10928)?
I don't believe there is any significant functional difference between these parts in terms of what can be built with them. The 10928 variant is simply a reinforced version of the original.

This gear is widely used, but because of its small size, it has always had problems with breaking under high torque. It looks like this new variation is attempting to address this by reinforcing the teeth so that they don't snap off as easily.

Similar minor strengthening changes have been made to other gears as well. Here's the 16 tooth reinforcement for example:

 

TLG has been known to make this type of structural change to lots of other parts as well. The most prolific example is probably the 1x1 plate with vertical clip that has seen a number of changes to address structural issues: