I recently worked with a client who had me convinced I was losing my mind as what I saw and what they saw was completely different. It took us awhile to figure out I was designing for desktop first and they were viewing the site on a tablet. Once I learned this it allowed me to shift to design for mobile first.

What does this mean? Most CMS tools are now "responsive" which means they take into consideration that more and more people are viewing websites on their phones while on the go. Your site has to be mobile friendly and if not Google will penalize you for it in your ranking and SEO.

When I design, I typically design desktop first then review the transition on mobile but if you know your clients are viewing primarily on mobile, maybe you should do this in reverse. If this is the case, not all CMS tools are the same.

Squarespace is responsive and you can view tablet and mobile views throughout the design process but they make the decisions for you. You can control the mobile view by changing the desktop design but you can not change the overall design, font sizes or hide lower priority items without custom code.

Weebly offers certain themes that are responsive but again you have little to no direct control.

Next up: WordPress, Wix and Editor X. If mobile is your priority these may be the better choice as each of them allow - more accurately insist - that you design specific to mobile. They all allow you to move or hide items, change font sizes, etc. specific to mobile without disturbing your desktop design.

Bottomline: If you want ultimate control of your mobile UX, WordPress, Wix or Editor X are great choices but if you want a tool that takes care of the design decisions for you Squarespace is the way to go.