AMP is more suitable for content sites/blogs. E-commerce portals / sites supporting transactions should go for a PWA. Here's why -
Accelerated Mobile Pages enable websites to load the content as fast as possible. The AMP framework removes all the irrelevent scripts from your website to save on bandwidth and focuses only on the content part. Heavy images/elements are loaded only after users view them. It helps content sites rank better in SERPs.
Progressive Web Apps enable websites to create a native app like experience and support features like offline experiences, background syncs, push notifications. Ability to handle offline experiences, processing certain requests in background using service workers make PWAs better suited for E-commerce.
There's a fantastic answer here: on quora that answers this question, but since Shopify isn't supporting AMP natively, the best alternative to developing your own AMP theme is to install a plugin that does this for you. AMP by Shop Sheriff is by far the leader in the apps ecosystem for this.
You'll be able to activate AMP for each piece of your store, and customize it quite heavily to suit your needs, your store theme, your product reviews, and some other extras. The SEO benefits have been astounding, and are easily visible in Webmaster Tools (you can filter by page and see position of AMP vs non-AMP). If you haven't at least tested AMP out, you probably should.
If you have any questions surrounding this, I'll be happy to help.
Shopify isn't supporting AMP natively
That's not quite true - unless you're excluding theme modifications. You can build an amp page on Shopify without out an app.