Succinctly, the difference between Cain's harassment and Obama's is that multiple women have credibly accused Cain of harassment and zero women have credibly accused Obama.
For the record, I would not enjoy being called "sweetie" by a man I didn't know, but it is not harassment, in and of itself.
Yes, but sexual harassment is a vague term, so Cain may have only called a woman sweetie, and that could have compelled her to bring up the charge. We really need to know the details behind the charges.