It's each to their own really. If you have to ask the question, I'd say you don't need a PS3. I have all major consoles, but I spend 99% of my time playing on the Xbox 360. That's just my personal preference. I keep my PS3 for when there's a lack of 360 games I want, and I can then pick up some of the PS3 exclusives for a cheap price and play them then (inFamous is worth owning a PS3 for on it's own IMO).
As I said though, if you have to ask the question, you probably don't need a PS3. Wait until you feel you do need it.