There is another factor to consider. Both airframes will often fly in "international waters" just offshore using high angle offset photography and other types of reconnaissance. If the "belligerent land owner" under observation becomes aware that some very sensitive information has just been gathered, he might want to take a shot at the offender. There are greater penalties to be paid for shooting down a manned aircraft then for a drone. Besides the U-2 can fly higher and gather more information.