Both can be used to create a responsive website.Responsive Layout Maker
is great if you're the kind of designer who likes to get "under the hood" and work with code. RLM solves one of the biggest challenges in responsive design, having a flexible core which can adapt based on the viewer’s screen size. In a nutshell, Responsive Layout Maker creates a responsive prototype website (called a layout). You build the structure of your website in Responsive Layout Maker and then export to any code editor
to add your final content and design styles.Responsive Site Designer
on the other hand, allows you to add the design layer visually
without having to dive into the code. These design tools allow you to add background images, gradients, shadow, radius, and set state functionality (hover, active, visited). There's more cool stuff like a web inspector tool, adding meta data and footer code, integrating and customizing font icons (1200+ icons come with the app) and more.
Get more details on the comparison here: http://www.coffeecup.com/help/articles/1423