Magento CE can be just as good as EE with the right performance tweaks, but this means getting a good agency to make sure that your site can meet a certain standard. You already have numbers, so you should request a performance test for the finished implementation, and account for a few sprints that include performance improvements. EE can be bogged down by bad code/setup just as well, so taking the time to test will yield better results.
Feature-wise, I'd say look at the features of EE and if there's something you need for launch, and can't find a 3rd party module that does it, it may make sense to go EE, as custom development tends to cost more.
Re: shopify, it costs way less but has limitations. It's a good proposition for those starting out but there's a point where you need the flexibility to edit code, or tweak performance, or adding unique features. One example is multi-currency, which Shopify can't do (afaik).