I did research on this a while back because I had a similar question.  What I learned was that Zyrtech is an antihystamine meant to block the allergic responses in the upper respiratory tract and eyes caused by hystamine, and Singulair a leukotriene blocker meant to prevent the allergic response of bronchospasm and wheezes caused by leukotrienes. Since both tackle allergies in different ways, some patients find a benefit from being on both Singulair and Zyrtech (or Clariton).

To add to this, some studies that showed that Singulair works well enough alone that Zyrtec (or Clariton) is not needed.  Other studies show some patients benefit from both.  So, agian, which medicine works best is basically determined on a trial and error basis.  I think most doctors would have you try Zyrtec first and if that doesn't work trial you on Singulair to see how that works.  However, which route you should take depends on you and you physician.