Isn't journaling (what a ghastly word) a bit more unstructured and free form than traditional diary keeping? I think of keeping a diary as writing things that happened in a traditional format, whereas journaling is more in the way of random thoughts, ideas, jottings etc in a kind of spontaneous way.
I've not kept a diary since I was a teenager, and I don't think then ever entirely successfully. Usually a new year's resolution which petered out by the end of February.
I kind of like the idea of an app, though. I do use notes on my phones for various things, but perhaps an app may be a better way of doing it. I'd probably only use it 2-3 times weekly.