Sony 18-250 vs. upcoming 18-135

Total Amateur Here - could someone please explain to me the advantage of the upcoming 18-135 vs. the 18-250. I am planning to purchase the a57 with one of the lenses.

From what I can see, they are both similar size and weight. I'm a total newbie, but what am i missing? What is the disadvantage of the 18-250?

Thanks in advance