Depends on what you enjoy doing. I spent a lot of time doing magnetic simulations during the earlier period of my PhD and the reason for doing them isn't that the theory can't explain what will happen, but rather that we don't have the mathematical chops to actually calculate it, so we build little artificial worlds and try to take measurements the old fashion way.

This was certainly a lot fun, but you won't be getting any thing particularly revolutionary out of it. I remember my advisor at the time running a few dozen CPUs of our poor excuse for a cluster (built mostly by yours truly with off the shelf components) for the entire summer to add a couple of digits to the ground level energy estimate of a completely artificial magnetic model :)

On the bright side, it taught me a lot about programming, numerical simulation and optimization