If you are only doing it because of the ammo available, then no, it's not worth it. Spend $10 on shipping for the .308 ammo instead of $800 for a gun and scope. If you are looking at getting a different gun for another reason, then by all means, do it! Can never have too many!
If I were you and looking between the 30-06 and .300 WM, I would go with the 300. It will give you something that is different from the .308 you already have, whereas the 30-06 is going to be somewhat similar when it comes to velocity, bullet drop, etc.