Different products wearing the same word
Squarespace sells coherence: curated templates, hosted everything, an editor that is hard to break. WordPress sells depth: any feature any developer can build, any host, any direction. One optimizes for the owner who never wants to think about software; the other for the business that will outgrow decisions made in an afternoon.
Squarespace wins when
Visual brand is the product (photographers, restaurants, portfolios, events), the page count is stable, commerce is light, and the owner values "cannot be broken" over "can be anything". Its templates are genuinely well designed and its checkout is genuinely fine for simple stores.
WordPress wins when
The requirements list contains any of: custom post types or data models, complex commerce, membership systems, multilingual at scale, business-system integrations, content operations at volume, SEO as a competitive channel, or ownership that survives any vendor mood swing. Also the long-run cost curve, like Wix, the hosted monthly compounds while WordPress plateaus.
The walls, named plainly
Squarespace walls: no real plugin ecosystem (per-feature apps instead), API access only at upper tiers, export gives you XML (content, not the site), and platform rules apply, one policy change and your checkout flow changes. WordPress walls are self-inflicted only: unmaintained plugins, no backups, cheap hosting, all solvable with a maintenance discipline.
If you are on one and need the other
Squarespace → WordPress is a standard engagement with a standard answer: content ports cleanly, design rebuilds, SEO gets redirect maps. Going the other direction is rarer and usually means "the site was over-built", worth admitting early. Undecided? Message the specifics, the answer is usually obvious within one exchange.