" As bad as AT&T is with layoffs, at least I know it will still be standing ten years from now..."
That could be true. However, you , I , or many of us may not be in that picture.
Your value here, and time with this company is on borrowed time. Longer or shorter.
Depending on your age and a whim of your manager / director.
It comes down to what you really want. So called stability ? or do something for yourself. You will never have the chance to grow as much as you may in a startup.
I for one, if I would not hesitate a moment. The direction T is moving, is not good and it doesn't look it will change any time soon.