I'm actually doing both. Found a great systems biology program where I have co-advisers. One is strictly theoretical modelling while the other is strictly experimental. Collaboration with both is allowing me to experimentally test/back-up my models! However, if you want an opinion on one or the other.... Modeling has an advantage in that you can publish papers much much faster. There is no experimental work. However, this leads me to the disadvantage that you have no experimental work to back-up your claims. Therefore, a lot of your criticism from reviewers will be about it being theoretical. Also, because you have no experimental work, your papers most likely will be lower impact which isn't good. On the experimental side, you will probably have higher impact but reviewers will be much more critical of your techniques. Also, experiments take forever and you need a lot more funding for them.