Surfer SEO, at $49/mo against $139/mo.
Semrush vs Surfer SEO
Surfer SEO is cheaper at $49/mo, against $139/mo.

All-in-one SEO, content, and competitor tracking
Content editor scored against top-ranking pages
Semrush and Surfer SEO are alternatives in the SEO category. Semrush: All-in-one SEO, content, and competitor tracking. Surfer SEO: Content editor scored against top-ranking pages. Surfer SEO is cheaper at $49/mo against $139/mo.
All-in-one SEO, content, and competitor tracking. It sits in the seo category.
Content editor scored against top-ranking pages. It sits in the seo category, and ships in 1 tested stack here.
They are alternatives at the same step, so most stacks pick one or the other. No aistack.sh stack uses both today, though nothing prevents a custom workflow.
Beyond each other, Semrush's closest alternatives are Ahrefs. Surfer SEO's closest alternatives are tracked in its tool page.