I have really no way of knowing which is more difficult, but it doesn't make a difference. Bloomberg has no national presence. He could never unite the conservative Republicans, and liberal Democrats outside New York would be suspicious of his party affiliation. I could see him possibly running for Senate, but given his experience and talents, the executive branch just makes more sense.

If memory serves, the last time a retired General became President was Dwight Eisenhower. Incidentally, if a Republican like him ran for President today, I'd probably vote for him. He once wrote "There must be no second class citizens in this country."

The point is that a Mayor doesn't have the national presence that a General does, and Bloomberg has other specific disadvantages in a Presidential race. He will probably make an excellent Governor, someday.