MK-0524A includes MRK’s proprietary anti-flushing compound, which is much more specific than OTC anti-flushing agents such as aspirin, according to Dr Kim.
In the early 90's, Kos and Upsher-Smith both came up with the concept of a single daily dose of extended release niacin dosed at night. The companies came to a cross licensing agreement in 1997 (Upsher had the first patent app but Kos claimed they invented it first). Upsher ended up selling the rights to Slo-Niacin (or Niacor SR) as part of their K-Dur settlement and Schering scuttled it after Niaspan's disappointing launch. They had completed some Phase III work but were a few years from approval.
To answer your question, the two products are very similar. Kos has made a bucketload of money by convincing people that their release is somehow unique (and your liver will fry if you take anything else) but if you look at the patents some OTC formulations actually release the niacin faster.