If one's aiming for an objective judgment, avoiding as much subjectivity as possible, it's the PS3.
It did sell marginally more in the end, and it's library ended up having more high-rated games. That's considering the first 2 years draught on the PS3 x last 3 years draught on the 360.
That's interesting, especially considering that the 360 had one more year on the market, since it launched in 2005 and the PS3 in 2006.
So, any other argument is completely subjective: PS3's PS+ great value x XBL more active gaming community (especially in the starting years of the gen) - free online x overall better online service - privileged game genres in each platform - hardware strenght (really? if there was a gen where this ultimately didn't matter that much was on gen 7, more than any other) - price (not a point if a gen ended, since it's only a means to obtain a result: selling more than the competition), etc. Any of these points can go either way, it just depends on how you view it.
PS: Not really counting the Wii here... or it would have won sales by default, while the PS3 would win on exclusives. Then there's subjectivity all over again, if sales matter more than library when trying to declare a winner, and vice-versa.