SEO
Both platforms are competitive on SEO. Squarespace has a clean URL structure, automatic sitemap, automatic image alt tags, and structured data for products. Wix has similar features, plus the Wix SEO Wiz which walks through the setup. Neither is as good as WordPress for advanced SEO, but both are good enough for most small sites.
Blogging
Squarespace is the better pick for content-driven sites because the editor is designed for long-form writing. The blog post editor, the categories, the tags, the author pages, and the related-posts feature are all in the box. Wix has similar features, but the blog editor is more rigid and the layout options are fewer.
Mobile apps
Both platforms have iOS and Android apps for editing on the go. Squarespace has a more polished mobile editor with the same design language as the desktop. Wix has a more functional editor with more options, but the design language is harder to maintain on mobile.
Customer support
Squarespace has email support and live chat during business hours, with a strong help center. Wix has 24/7 phone support, a strong help center, and a community forum. The two are close on support quality, and the answer is more about which platform you prefer to use.
Real-world use cases
The right pick depends on what you are building. A photographer or a designer who wants a portfolio site with a strong visual identity will be happier on Squarespace because the templates are designed for that audience. A restaurant or a small retail store that wants a site with a few pages and a simple store will be happier on Squarespace for the same reason. A service business that needs bookings, quotes, and a more flexible layout will be happier on Wix because the editor allows the same kind of customization that a small WordPress site would allow.
A content publisher, an artist with a shop, and a creative professional who needs both a portfolio and a small store are the right use cases for Squarespace. A small business that needs to add an unusual app like a calculator, a quote form, or a chat widget is the right use case for Wix. The two platforms are converging in features, but the design language and the customization model are different enough that the right pick depends on which fits the project.
Migration between platforms
Migrating between Squarespace and Wix is supported on both sides, but the result is rarely as clean as the original. The text content and the images transfer, but the page layout does not. Most users who switch rebuild the site on the new platform, which is a half-day to a full-day project depending on the size of the site. Plan for the rebuild rather than the migration, and choose the platform the first time based on the design language you prefer rather than the migration story.
The honest short version
Squarespace is the better pick for designers, photographers, restaurants, and content sites where the design matters. Wix is the better pick for small businesses, small stores, and buyers who want flexibility and lower-cost entry. The two are close on features, so the answer is more about which design language and which user experience you prefer.
Photo: Pexels, Pixabay Content License, via Pixabay (https://pixabay.com/photos/chairs-conference-room-contemporary-2181994/)