Short Story vs. Novel

I have many ideas for potential stories, but I have recently run into the problem of trying to decide which ideas will be short stories and which will be more novel-length stories.
This is difficult for several reasons, first is that I've never written a novel-length story before. Second is that it's difficult to figure out which ideas are even capable of being long, and which cannot be lengthened without becoming boring or redundant. Third is that it seems some ideas aren't even worthy of being long.

Here are two real examples of simple ideas I have in the works.

1. A person finds a way to never fatigue/gains the ability to never wear out.
2. A person begins having recurring dreams of killing someone close to them, the dreams may or may not eventually begin occurring while awake. The person must figure out how to end the visions/dreams.

They are both very simple ideas that both seriously need to be "fleshed out" in order to do anything at all with either of them. The first should be pretty easy to run with, after all the possibilities of what to do with such an ability are endless; from seeing how it affects mundane day-to-day activities, to how it helps in the event of some disaster.
The second idea I can see myself running into trouble with. It certainly sounds interesting enough to make novel-length, but I don't know if it's possible (for me) to do. It's a very specific problem, and the main character with the problem may therefore not be dealing with it all the time, so what keeps the story interesting while they aren't having the visions/dreams? Clearly this idea will take a lot more thinking out in order to make longer than a short story.
Then again, there's no shame in writing short stories. Maybe I shouldn't try to do something I'm not comfortable with simply out of fear of short stories being less popular or taken less serious.

Besides, short stories make better movies.
-Ryan