There is an important litterature on this point. This paper (When should we use one-tailed hypothesis testing?, http://onlinelibrary.wiley.com/doi/10.1111/j.2041-210X.2010.00014.x/pdf) is simple and basically says that (in agreement with several others):
adoption of one-tailed testing requires an explanation why the authors would treat a large observed difference in the unexpected direction no differently from a difference in the expected direction that was not strong enough to justify rejection of the null hypothesis.
In other words, you cannot use one-tail test if you are only interested in difference in one direction but only if you are convinced that difference in the other direction is "irrelevant" in a certain sense.
For example, if you suspect that a given parameter is superior to 0, you cannot simply use a one-tail test. However if phyisically, this parameter cannot be negative (let's say that you measure a kinetic energy which is by definition positive) then you have the "right" to use one tail tests as measuring a large difference in the negative direction is just irrelevant with the current physics.
Another example : suppose you have some water samples in which some are added a solution that is known to increase the magnetic property of the sample. You want to test for each sample if the magnetic property is increased or not to test the presence of the solution. In such a case, I would say that you can use a one tail test because physically having a descrease of the magnetic propery is irrelevant and can only be interpreted as measurement errors (at the current state of knowledge).
I also suggest this paper (http://www.bio.sdsu.edu/pub/stuart/2009MisprescriptionOneTailed.pdf) that contains tons of reference.
To conclude, I would said that, according to these recommendations, in your case a two tailed test is needed.