In my opinion: Buy a dual protective case, something that will be impact resistant and protect those flimsy corners...
Galaxy Ace comes with Gorilla Glass, it will resist scratches a lot more than regular screens.
Screen protector is OK to protect the glass, good protectors wont leave residue on the glass, and if they leave some residue, you can wipe it off with soft solvent. Also replacing the glass is inexpensive now days, unless you got a latest model...
Mayor concen on most of the Samsung phones now days is breaking the glass. It is true they have gorilla glass, but since the frame of these phones is made of plastic, when they fall from your hands to the concrete floor and land on a corner...... Ouch!!!! Glass breaks.
It kinda sucks that the flagship phone of Samsung, the Galaxy S3, suffers from the same defect. Its a flagship phone and they couldn't put it in an aluminum frame?.... /sigh