what differences are there between the macbook pro 15'256 and the 516?

If you go to the following page and first select either the $2399 or $2799 model, you can then independently select

1 - 2.8 or 3.1 GHz processor

2 - 256G to 2TB Storage

3 - 555 or 560 graphics

If you select the $1999 model there is a choice of 3 processors and a choice of storage but no graphics choice. You can view all of the above choices without proceeding to the actual purchase step.

The web page is: Buy MacBook Pro - Apple

In addition to the storage difference, the processor and graphics. See the following: MacBook Pro - Technical Specifications - Apple

You can't upgrade these so buy what you need.

256gb SSD for basic storage

512gb SSD for Dual boot boot camp OS X/Windows

Larger SSD storage for Music and Videos.

I think the base model 15" MacBook pro with 512gb of storage is the best value deal out of all the MacBooks.