The best answer here is - it depends. If your ecommerce platform is an open-source one, then having a good back-end developer may be more vital. However, it’s still likely you will need both. If you’ve chosen a SaaS platform, then you probably require someone who’s able to do more of the front-end work. However, you will still need a person capable of building APIs since even SaaS platforms need to be handled by an expert if you want to do more complex customization.
All in all, if you genuinely want to create a unique website, it’s likely that you will need to work with both front-end and back-end developers or with someone capable of doing both on a high level.