While they are both named Shopify, underneath they are vastly different platforms.
Shopify Plus is designed for enterprise-level eCommerce businesses and can handle thousands of transactions per minute. (As an example, look at JB HI-FI and Patagonia.)
Shopify Plus pricing starts at $2000USD/month and you get a lot of enterprise capabilities for your money.
If you run a significant online business, Shopify Plus might be perfect for you. Concise Digital is an experienced Shopify Partner, and we can talk you through the options so you can make a fully informed decision.