The concept of website builders and content management systems are becoming more and more intertwined as we continue to move forward through the mobile age. Since businesses are becoming increasingly Web-savvy, the tendency is to make the majority of the decisions concerning their site, hence the high demand for inline content editing and the contrast of sitebuilder versus CMS.
While website builders are often hosted in the cloud as a Web-based application (designed to make things as easy and tech-free as possible for the end-user), content management systems today see a fairly even split between self-hosted and SaaS (sofware as a service) models.
For the creators of these software, both website builders and content management systems alike, much of the application depends on their monetization method. They can choose to sell it as a one-off product (with the optional reduced fee per upgrade, similar to what we would find with previous versions of the productivity apps we're all used to), or if it's hosted on their own servers (or by third-party Web hosts they're affiliated with)... the process is simpler, as your website now has an online home and place on the Net without any hassle.
Difference Between the Two
Website builders tend to use WYSIWYG (what-you-see-is-what-you-get) editors because their users for the most part prefer to have an instant, visual representation of what their site will look like as content is being created. Content management systems use a different approach, one where the content is kept separate from the styling and design functionality of the platform. The reason for the latter is that CMSs are often used by Web professionals to develop websites for customers that may or may not manage the content on their own. Either way, not much freedom is given to them for editing anything outside of text. Permissions can, however, be awarded for deleting or posting images (to list an example) - but this depends on the CMS in question.
In fear of a client "breaking" their design, digital marketing agencies will often manage a website in its entirety if a website builder is used to create it. This is understandably so as billable hours can quickly add up if an end-user with conflicting views on layout, etc literally touches something they weren't supposed to. And it happens to not be limited to design elements. System settings including account details can be mistakenly-changed, creating problematic issues beyond a website not looking right.
In truth, due to the growth in popularity of DIY (do-it-yourself) solutions in recent years, the lines between them and CMSs have definitely been blurred. Content management systems continue to require basic programming knowledge at the very least, but with all of the free and open source information available on the Internet, power-users are quick to get answers to their questions when they need them. Still, products that are marketed as website builders tend to facilitate the process further by not requiring any coding skills at all.
Sitebuilder/CMS hybrids are usually website-builder-first products that offer a type of developer platform for Web experts. The software companies behind them clearly don't want to leave any money on the table given the fact that their focus almost always rests on one side relative to the other. Although they gravitate more towards one end of the market (relying heavily on that niche for the bulk of their income), they try not to ignore the demands of the other group in hopes of gaining an extra set of users.
Regardless of whether you're a professional or a consumer, the direction you take should depend on your goals and what you're ultimately looking to create. As an example, a closed environment and proprietary system such as a website builder works well with small business owners because they're frequently packed with useful features out-of-the-box (like image galleries, form builders, calendars for posting events, etc). The downside, however, is that designing (let alone effective branding) can be both cumbersome and time-consuming, even if a tool-set is given for accomplishing this task.
For business customers with a marketing budget, hiring a developer to program a custom solution or one based off of an existing content management system may be a better option. The costs will be steep, but the power and flexibility will allow for a truly scalable website. As a consumer, the choice is yours, but choose wisely.
For specialists in the field of Web design, development, or marketing, an application is of less importance than the company behind it. If it primarily serves the consumer (i.e. the end-user), you may be better off utilizing a software that's unapologetically a content management system because CMSs (nearly all of which are extensible) are often developed by other agencies that understand your challenges, and will accomodate you in the tedious endeavor of working on elaborate customer websites. Today, more than ever, there exist CMSs with powerful and beautiful user-interfaces that make delivering dynaymic Web experiences a joy. You can have the best of both worlds as a professional.