Me, I only add original bottles to my collection (and preferably not decant bottles). Sure, if someone gave me a 100 ml decant of Aventus, I won't turn it down. But almost always, I'd rather have the original bottle.
Also, I only keep bottles that I absolutely love. I either decide whether it's worth being in my collection or if it isn't.
General criteria: I must love how it smells - not just like it. It must be wearable and versatile; something I could wear often and not just once every few months.
And on that basis, I either buy a full bottle - or I don't have any of it at all. This simply makes it easier for me since there are hundreds of fragrances out there, and there are plenty of borderline cases which are immediately resolved.