That's a very broad question. Graphic design is basically using images and text to present information, be it signs and logos, album covers, aircraft insignia, etc. Most often in 2-D formats. 3d animation is taking a modelled object and moving and warping it through frames, to create motion. Think anything by Pixar. A 3D animator doesn't usually do the modelling, but it's their job to get the niuances of the character's personality.
As far as what is better to further your career, they're too different to compare. It's a question of where your interests and talents lie, and what experience you've had in either field.