All superzooms (even "L" superzooms) have the jack of all trades syndrome. (Master of none).
I've seen lots of people with crop bodies that trade out the 28-300 for the 18-200, because the 18-200 is a more useful jack of all trades for crop. The only people I've seen that stick with the 28-300 are people that have full frame bodies, or people that have intention of getting a full frame body.
The problem with the 28-300 is that at 300, it is a 6.3 aperture. The 70-300 has a 5.6 aperture at 300. You will find that most bodies have AF trouble with the max aperture of a lens is less than 5.6.
What kind of photography are you going to be doing with this lens?