There is a big difference in Pro and Enterprise versions. Pro is a software which you need to install and host yourself on your server while Enterprise is a service provided to you. You need to pay for it on monthly/annual basis. You need to update the Pro version every time there is an updated version yourself and we provide 1 year support for each license you buy from us. While with enterprise version all the updates are done seamlessly, you don't even know that your service was updated and we provide support during your subscription period.
There is also difference in URL structure. With Enterprise version you will have a separate subdomain for each language which is better for SEO, besides it will allow to translate URLs of your site which is huge SEO advantage.
The free version in case of Pro or Enterprise will be your language selector only and when you click on a flag you will be directed to a language URL according to your paid version.
Our website is using GTranslate Enterprise version: https://gtranslate.io
Let me know if you have any other questions.