I'd use front facing, I wouldn't want to take a pic with my fav only to look at it and realise it was blurry or I looked a mess. Front facing cam quality isn't really that bad so I can deal.
I'd use the front camera with the square effect so I could cut my forehead off the photo lmao
But I have a 4S so the quality would be so crap and tbh I would probably use the back camera and I wouldn't EVEN care how I would look since it would be with my fav. Although tbh I'd smile and I think I have a nice smile so :-)
I always use the front camera when I take selfies with friends tho, idk why.