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Here are the fundamental difference between Pro and X Theme:
Demo Content: X Theme comes with demo content function. Because Pro Theme uses a different templating system, it does not has demo import feature like X Theme. However, Pro Theme has Design Cloud. To learn more, please take a look at following resource.
Header and Footer builder in Pro: The primary difference between Pro and X Theme is that Pro Thems comes with Header and Footer builder. Using Header and Footer builder, you can design global or page specific header and footers. To learn more, please take a look at following resource:
It depends on the requirement. Pro Theme comes with Header and Footer builder. While X Theme has Demo content. So, if you are looking to have a website in a very short duration, then X Theme makes sense as it has demo contents and especially Expanded Demo that can be imported to have a ready to use websites. Here's resource that you can take a look:
Both are SEO friendly and fully responsive Theme. At the same time, they have fast load time that also works in favour of the user experience. Page load time is also a ranking factor that search engines use to rank websites.
Both a fully compatible with WooCommerce. I would suggest you to list down the requirements and depending on the same you can choose the theme to website the website. I have listed the features that X Theme and Pro Theme offers.