That's an interesting question.
It's good that you asked and not just followed the gut feeling.
I have done both. Built something that I like and other products (WordPress plugins) based on some research or finding that somebody had a problem that needed solving.
Keep in mind whatever you pick you'll need to stick with it for at least 4-6 months to see some results. It also depends how much time you'll put into marketing to make it successful.
As I mentioned in my previous answers go to a local Startup Weekend event and look for potential ideas, co-founders, suppliers etc.
Ok to answer your question. Do some research in areas that you're most passionate about, research competition, find out how you can be better and different. Then create a STATIC site that shows what your app is going to be about.
Then go to potential clients (or attract them via ads) and ask for feedback....you can also ask them to pre-order....that will be a great validation.
And finally, make sure you feed you mind with good entrepreneurial content ( Mixergy, EO Fire, Smart Passive Income, Ask Altucher etc) so when the times get tough you'll be prepared.