Alright from using these two similar applications I think that Adobe Spark is the application. In spark you have more variety with themes and format. Also Spark allows your pictures videos and texts to be displayed in different manners. From a viewers standpoint and user standpoint I think that Spark is easier to use and more appealing to the eye.
Sway is still a good application to use, but compared to Spark the application feels limited. From using Sway it was slightly harder to navigate and not as creative with the set up of pictures. Also the template is all the same is is very boxy, not as much flow to it. Sway is still a good application to use for a presentation, but not for a project that needs a lot of creativity or style.
Overall, would recommend Spark!