You generally want to avoid dumping the air into the attic because it can lead to condensation.

[Link: human.cornell.edu]

Venting to the outside can cause a problem if it creates an air deficit. If the house is older you may be okay.

[Link: structuretech1.com]

When I first installed my fan (340 cfm), I dumped the air into the garage. I noticed massive condensation in the garage during the winter and also had a near constant sewer gas smell in the basement. I moved the vent to inside the house and those problems disappeared.