My Dr has told me that there's a reason that the brand name is more expensive.
While what I look for for my sinus issues is strictly pseudoephedrine and ibuprofen, most often the inactive ingredients are the same between Advil Cold & Sinus and store brand.]
If you don't get the sinus stuff from behind the pharmacy counter, then it's a whole different ball game. It's phenylephrine rather than pseudoephedrine... And my Dr has told me that most people have no relief from phenylephrine at all... No matter the brand.
With strictly Allergy meds....... There are several different types of base medication. Zyrtec, Allegra, Claritin... All 3 have different base meds in them.