Honestly, I don't think either of those will teach you anything that you will use often in the industry. I have had to implement GCD a couple of times, and I have implemented a couple of algorithms on graphs (union-find and topological sorting), but that's the kind of thing that you look up in Google of Wikipedia in about 2 minutes.
What will be useful in the industry (and in your general enjoyment of life) is the capacity to think abstractly that either of those subjects will give you. I would take both.
You are more likely to apply the kind of thinking required for Modern Computer Algebra, but on the other hand, Graph Theory might make you think of certain problems in a completely new way, which is a very good thing.