Primary difference between the income sections for form 1120 and 1120s Primary difference between the income sections for form 1120 and 1120s Solution Answer :- The primary difference between form 1120 and 1120s is that the form 1120 is filed by corporations on annual basis under internal revenue service (IRS) code whereas the form 1120s is filed by S corporation (corporation who have opted for \"S\" status) on annual basis under internal revenue service (IRS) code..