Journler and Scrivener are not really applications that deserve a direct comparison because they fulfill such different functions:

Journler is an excellent tool for keeping a diary/journal (it's original purpose), or just for collecting piecemeal information that you one day might want to use. I've used Journler on and off as a place to toss my story fragments and ideas that surface every now and again.

Scrivener, on the other hand, is focused on creating a single cohesive project. Although it has features that allow you to collect information similar to Journler, the things you toss into a Scrivener project are associated with only that project. If you're sticking text into Scrivener, then it's probably because you're trying to arrange it into a paper, presentation, novel, etc. and any MP3s or PDFs that you toss into the project are there for quick reference because they related directly to the project.

Journler is like the junk drawer where you just drop whatever random thing is on your mind, while Scrivener is the filing cabinet where you arrange things into logical groupings for later reference.

Off topic, I personally prefer Avenir (soon to be renamed StoryMill) for writing fiction. Scrivener's index card metaphor is cool and better for general use, but it's one of those programs that tries to include everything and the kitchen sink, which doesn't work for me (and I really dislike its lackluster annotations). Avenir is more structured, but if the structure makes sense to you then I think it actually is more effective at keeping you focused on writing. If you're looking for software to help you in your writing, definitely try both.