As someone who has used muse a bit I can say that the differences are:

RWD - muse also has this but not as good; it creates seperate html files for each variant and is limited to two variants.

Dynamic - Muse is completely dynamic; it has designated header and footer areas and the page expands according to demands of any dynamic code, so for instance ecwid pages will shorten and lenghten accordingly.

Parallax - Think of any parralax effects and muse can do them, fading parralax effects, animated parralax effects etc etc.

Widgets - the community can create widgets such as slideshows or button effects (such as fading rollovers) and you can install them into you library, and there are already hundreds created since the launch of this feature in November 2013. All widgets such as slideshows or tabs or accordians are fully customisable (size, colours etc etc. Widgets include full screen slideshows.

Rollover effects - there is a dedicated States option which means you can create inactive state, hover state, active state, visited state for anything without resorting to layers.

Forms - built-in form creator with the form backend hosted on your own servers (must have latest php) Every aspect of the form is customisable.

CMS - there is a full cms built in which allows a client to update anything you let them and all styles (fonts for headings etc) are kept, the interface for the control panel is similar to wordpress but fully wysiwyg. At the moment this feature is only available if you take Adobe Business Catalyst hosting (you get this free with the subscription) but according to reports is coming soon no matter where the site is hosted.

If you are used to any adobe products then you'll be able to use it to create a site within hours, if not then it's a couple of days learning but remember that the adobe muse community is far larger than xaras.

I had got to 98% completion of my own site with xara when I decided to go with muse for it's potential to creat stunning creative sites but then xara made it difficult for me and went and released v10, and to be honest (baring some teething problems with the rwd) I will go with webdesigner simply because the route they have taken with achieving responsive web design is far better, google recommend that only one html file is created for each page and webdesigner does this, whereas muse creates three separate html files, so for seo purposes it is far far better. I will still be using muse for some dynamic sites which require ecommerce.