my impression is that apps are toys, useful for snaps or funny effects, but if you want to seriously look at editing your photos then pc software is the way to go. i have been using paint.net (it's a free piece of software to cut your teeth on) but have just ordered the latest version of photoshop elements (like a cut down version of the real thing that costs 1/10 of the price!!) to get properly stuck into it.
i am NO expert on this, having vehemently hated photoshop for many years

for the OTT effects and manipulations i've seen. i can't stand photos of people airbrushed within an inch of their life, and always used to get really frustrated that the photos i took of things never looked like the ones i was aiming for. then i got off my high horse and realised that almost every shot i see has been edited in some way, and if i want to achieve that sort of shot i'll need to get into it.
mind you, i still can't stand airbrushed models...!