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If it's just for viewing, I'd go with a 3D PDF. Most likely your client already has Adobe Reader installed on their machine, so you wouldn't have to help them through a learning curve on a new viewer.
Just do a "SaveAs" on the file and select PDF U3D as the output.
Be aware that they'll have access to all the 3D info. (Since you were looking to send STEP or IGES, I'm assuming that IP isn't a problem)