For any product what should I choose a separate product page or just side bar? Obviously it will have a separate sales page that's not the issue here.
Without seeing you site or product its not easy to suggest anything. But I will say the try them both one at a time. See what works best for you.
If you already has a separate page then both can turn out good. You just need to see which one works for you better.
Separating product page will depend. Products that has similarities or usually versions will take one page while for the side bar will be the top sellers.