Should i take montek lc or desloratadine in terms of effectiveness and less sideeffects for long term use. i have allergy for past several years?

Depends: It depends on which one works better for you since they are both relatively safe drugs. Usually a nasal steroid will likely be more effective. See an allergist if problem is not controlled by medications.

What kind of allergy: do you have? Is it respiratory tract allergy? upper or lower? Is it truly an allergy problem or something else? Lot of questions as you see, and no simple answer. Montek LC is a combination medicine (montelukast and levocetirizine) an anti inflammatory and antihistaminic 2nd generation, desloratadine is like levocetirizine; u can take either for long term, but better C an Allergist / ENT, goodluck