It's not really a one-or-the-other question.
On-site SEO should always be there. It's so fundamental/ingrained that I'd have to actively work to avoid it even for the most independent/experimental landing page.
But even if you have a full-fledged retail/business website, your PPC campaigns are going to be highly targeted at domain.com/product-name/landing-page-B.htm and many more longer-tail pages. You definitely want to be pushing backlinks for your domain.com/product-name/ pages, but your landing pages might not be targeted enough if it's viable for you to be spending resources on SEO for them.
I also generally have a different front for organic traffic and paid traffic.