Ultimately you should learn both, but just pick whichever you feel most comfortable with to start out.
It's far more important to not overreach yourself when starting out. E.g, and in the case of OpenGL, you sometimes see statements along the lines of "you should pick OpenGL because that means you can target Windows, Mac, Linux, PS3, iPhone, Wii, etc". Aside from being false, this is one of the worst reasons to choose an API when starting out. You need small achievable goals, and wanting to hit every platform under the sun is neither small nor achievable for a beginner.
Instead make your choice based on which API's syntax pleases you most, which one looks as though you might be able to start getting stuff done easiest in, which one is most appropriate for the platform (and make it one platform) that you want to learn on. Learning should be fun, joy, exploration and excitement, and with a sense of genuine achievement at the end, so your choice should reflect what it takes to get you there.