One of these is a commerce platform that will let you write. The other is a publishing platform that will let you sell, awkwardly. Which way round you need it is the whole comparison.
A shop with a site, or a site with a shop
Shopify handles tax, shipping and inventory properly rather than approximating them, and its app market covers trades the core product does not. Shopify scores 5 for room to grow against WordPress.com's 4, and the gap is entirely about volume and complexity of selling.
WordPress.com is the managed version of the software a great many websites already run on. Plugins are available from the cheapest paid plan, content exports in a file another WordPress site will read, and a free domain comes with paid plans. For a site whose main job is publishing, with a few things for sale attached, it is far cheaper and perfectly capable.
Shopify scores 4 for ease against WordPress.com's 3, which surprises people — a commerce platform is more opinionated, and opinionated is easier when the opinion fits.
Where the percentages hide
Both have a number that is not on the price page. Shopify's promotional starting rate rises steeply once it ends, and using an outside payment processor adds a percentage to every order — on a shop doing real volume that second one is the larger cost by a distance.
WordPress.com's advertised entry rate lasts twelve months and then renews at roughly two and a half times as much.
Shopify scores 2 for value and WordPress.com scores 2 as well, which is unusual for two products this far apart in price, and it is because both are cheaper at the point of sale than in year two.
Shopify staffs support around the clock. WordPress.com scores 2 for support against Shopify's 4, and has more concepts to learn than a drag-and-drop builder.
Which should you choose?
Choose Shopify if selling is the business, or if tax and shipping are already more complicated than a flat rate.
Choose WordPress.com if the writing is the business and the shop is incidental — and diarise month thirteen, because the renewal is the part people are surprised by.

Shopify
Sells at a scale the friendlier tools cannot reach, and prices for it.

WordPress.com
The managed version of the software a great many websites already run on.
Specs
Ratings
Ease for a beginner
Design and templates
Room to grow
Help when you are stuck
Value for money
Features
Pricing
Shopify
WordPress.com
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