I would bet on this one since it's based on a need from your knowledge of the industry. Now you have to validate if it's a really important need and someone would pay for the solution.Second one is something very useful for the industrial marketplace. I work for a company called Fastenal, where we sell industrial and construction supplies. Spotted a need that was difficult and thought up a way to solve it. Thought about also trying to pitch this idea to the fastenal company itself to sell them the app idea for a royalty. The app is something every consumer and producer in the construction industry could use.
Idea selling doesn't work in most cases. The cheapest way you can do it is to partner with someone who knows how to code an app or get some investment money which would be insanely hard with just an idea.Long story short, I am considering just selling these ideas for a royalty rather than going through the long process of making the app etc. I have no clue how to make an app and would most likely outsource it which seems like it could get pricey.
Good Luck!