The 16-35 and 24-70 are both good lenses for full frame. Your choice should be determined by what angle of view you want to use most often. If you like "normal" on your 50D, then go for the 16-35. If you prefer "telephoto" on your 50D, then go for 24-70. (It will change when you get full frame.)


That said, are you aware that by restricting yourself to full frame lenses that you will pay over three times as much? And that even after paying so much more, the image quality will actually be worse?


For example, the Tamron 17-50 f/2.8 is $450, while the EF 16-35 f/2.8 II is $1,500. But the Tamron is sharper, longer, and lighter. That's why I consider it a mistake to restrict yourself to EF lenses just because there is a "possibility" that you will upgrade to full frame in the future.


There is another complicating factor here. On a crop camera, the best EF lens in the "normal" range is the 16-35. But on full frame, the 17-40 becomes a much better deal, because f/4 gives you the same DOF as f/2.5 on APS-C, so you no longer need the f/2.8 like you did with a crop.


Hope that helps.