The key differences between helloSystem and Étoilé are their philosophies and underpinnings. My understanding of helloSystem is that it's primary concerns are ease-of-use and UI consistency. It's aiming for the ease-of-use of the classic Mac OS with the UI design and polish from the Tiger era of Mac OS X. I haven't tried it yet, but I've been keeping up with its progress and it looks very nice. helloSystem is not based on GNUstep or any other Apple-inspired APIs, however. Rather, it uses Qt as its GUI toolkit.

Étoilé was an attempt to bring a refreshed desktop experience that is based on GNUstep yet isn't a reimplementation of the NEXTSTEP or Mac OS X user experiences (though there are heavy influences), introducing its own concepts of what makes an ideal desktop environment. In addition, Étoilé is very aware of the Xerox PARC influences of NeXT, being cognizant of the notion of NEXTSTEP being a pragmatic Smalltalk OS. With this in mind, Étoilé supports a Smalltalk dialect called Pragmatic Smalltalk, which is able to leverage Objective-C libraries and thus fully integrates with the rest of the Étoilé system, which makes it quite different from the experience of running Squeak or Pharo in a window on top of a host operating system and addresses one of the primary complaints some prospective Smalltalk programmers have about Squeak and Pharo. Pragmatic Smalltalk is built on top of Étoilé's LanguageKit, which allows for the development of dynamic programming languages that can interoperate with Objective-C. There's also an EMCAScript implementation built on top of LanguageKit.

In my opinion, if Étoilé were still active, it would attract a different type of user from helloSystem. While both helloSystem and Étoilé are heavily inspired by the Mac, helloSystem is focused on recreating the Mac user experience, while Étoilé was focused on extending the NEXTSTEP/Mac OS X power user/developer experience.