If your hypothesis is ‘greater than’ or ‘less than’, use a one-tailed test. If your hypothesis is ‘different than’, use a two-tailed test.
The way to code it (for ttest2), is for example to test that the mean of ‘x’ is less than the mean of ‘y’:
[h,p,ci,stats] = ttest2(x, y, 'Tail','left');
Examine your data and consult the documentation to determine the correct option.
