It doesn't matter how fabulous your storytelling is if your users can't find what they are looking for. Exceptional UX feels comfortable, intuitive, and... unmemorable. Your site visitors shouldn't have to think very hard about how to consume your content.
How easy is it to crate a navigation path for visitors to consume your content in an intuitive way? It's more than just adding and editing menus and buttons.
This is one of Wix's main drawbacks. Navigation only goes to 2 levels. But it's easy to edit all page and navigation attributes within the menus and navigation option on each page.
The navigation interface is a little less intuitive in SquareSpace than Weebly and Wix, but it is possible to add subpages giving you two levels of main navigation.
Unlimited levels of navigation - which can be good for larger websites, or worse for user experience if it's not well thought out.
Depends on the theme, but WP can support the most complicated of navigation requirements. Know your navigation requirements in advance, and choose wisely with your theme selection.
Apps & Plugins
Is there an app for that? If the functionality isn't already built into the platform, can you add it? Some have app marketplaces similar to the iTunes or Android App Stores you use to customize your mobile phone.
Wix has an AppMarket with plugins you can add to your site for additional functionality. Some of the apps are Wix-developed, some are from their developer ecosystem; some are free and some are an additional cost. They have basic marketing tools that make it easy to manage light email marketing, contact management, chat, and social. You can find a growing number of apps available for common web-related needs, like disabling data sharing (CCPA compliance). Many of the software providers you're already working with have integrations - like Hubspot, QuickBooks, Google Analytics, and more. Research your apps though, some work great and others, well, not so much.
SquareSpace has built in all the features they think you will need automatically into their interface, so there are no additional apps to install. You can use Zapier to connect to your tech stack, but that's a heavier lift than most companies who are a good fit for SquareSpace to consider.
Weebly's App Center is similar to Wix's, extending the functionality of your site. In contrast to Wix, the majority of their apps are design-centric, where this functionality is already baked into the Wix design interface. Similar to Wix, some apps are very strong while others are just plain broken. Both paid and free apps are available.
There are countless plugins available for the WordPress platform, some paid, some free. You have to do a little more vetting of plugins with WordPress that with the website editors since it the marketplace has a less oversight and control. Some plugins are paid, some are free. All websites are vulnerable to security risks. Because many WordPress sites leverage external templates and plugins, it's extremely important to make sure that you stay current with updates.
Thanks to companies like Square and PayPal have made it easy to take payments online, letting just about anyone sell online.
Verdict: SquareSpace, for now
Easy-to-manage product management for your store. Customer accounts are a recent addition.
The Advanced Commerce package has more robust features than Wix and Weebly, including shipping, discounting, abandoned cart recovery, inventory levels, and gift cards.
Easy to upload your product list, calculate sales tax and shipping costs. Weebly was acquired by Square, the payment processing company, so you can assume that they are going to double down on ecommerce capabilities.
Depends on the theme, but WP can support any ecommerce, at scale. Again - choose wisely with your theme selection.