I'll state up front that we are a Shopify partner,  so obviously going to be slightly biased towards Shopify as an overall preferred platform for e-commerce. When it comes to ease of use however, we're a bit more objective.

Shopify is a powerful, purpose-built online business tool, but that means it comes with some of the baggage that goes with that tag. In other words, more features and more integrations can equate to more complexity.

With that in mind, it's really going to come down to which platform suits your business better. Ease is one aspect, but there's quite a few other points to consider before making a decision:

Design aspects. Both Shopify and Squarespace have free, basic design templates to choose from, so either is suitable for quick startup business if you want a stock standard design. If though, you need more of a point of difference or greater range of designs to choose from, then Shopify may be a better option, as there is a greater number of designs in the marketplace.   

Range of products. If inventory is small, then SquareSpace will be do the job nicely. If you have a larger range of products, then Shopify is the better option as it's made for scale.

Social Selling. Will social media be a large part of the way you do business and transactions? If Facebook is your thing, then once again Shopify may be the preferred option. This is due to its direct integration with Facebook, where you can syncronise products bewteen your website and Facebook page, effectively allowing you to have dual shop fronts.

Summary

Both platforms can be easy from a getting started point of view. SquareSpace is the winner for smaller business and early stage, but for scalability and features then I'd lean toward Shopify.    

I think Mark has provide a well-written reply, but I'd like to add something else:

What do you want to achieve in the long run?

Both are quite easy to use (with Squarespace I believe a little more adaptable for beginners), but if in the future, you're planning to have a robust e-commerce platform selling hundreds of products, then Shopify my be your go-to. Compared to Squarespace, at the time of writing this post, seems to make it easier / more affordable to implement various payment options, such as multi-currency and Afterpay.

If you're looking for basic content or e-commerce (selling a handful of products), Squarespace, in my opinion, has been easier for beginning users with no experience making websites.