The verdict
Pick SEMrush if SEO is a serious channel for you and you want one platform for everything: its 26.8-billion-keyword database, backlink index, technical audits and competitor intelligence have no equivalent in Surfer, and the limited free account lets you test before paying.
Pick SurferSEO if your SEO is essentially content production. It does keyword research, rank tracking and the actual on-page optimisation brilliantly, starts at 49 USD/mo against SEMrush's 139.95, and gets you from keyword to a well-structured article faster. What it cannot do is the technical and backlink side, because it simply does not have those features.
For most lean founders publishing their way to rankings, Surfer is the honest first buy. The moment you need backlinks, site audits or competitor gap analysis, you have outgrown it, and that is when SEMrush earns its higher price.