Ultimately it depends on the type of metal, although almost all metals conduct electricity very well. A balloon can also vary depending on the material of which it is made. However, in general, a metal bar would be more conductive than a balloon.
You can compose a hypothesis by creating a statement. Do you think a metal bar is more conductive or a balloon? Whichever one you choose, create a statement that suggests that your choice conducts more electricity.
For example: A metal bar made of copper conducts more electricity than a latex balloon.
View the resources at the bottom of this question for more in depth information on formulating a hypothesis and properties of metals.
A metal bar as metals are good conductors of electricity and heat.