The Xbox360 and the PS3 are both excellent consoles for shooters, but for someone with one platform who is a big shooter fan the Xbox360 is still the best place to be, even if quality available is comparable.
The multiplatforms - CODIV, CODV, Bioshock, Battlefield Bad company + Many more.
The Exclusives for the PS3 - RFOM1/2, KZ2, Socom *Forgive me, im not up to speed on this title*,
Edit: Ok - MGS4 (Unique), i'll leave out the rest because I don't want to bring in too many more shooters.
For the most part, whilst the PS3 exclusives look to be high quality. They are very comparible with the multiplatform games out there. Its just more of the same. RFOM/KZ2/*Socom* have to compete directly with the multiplatforms available to get any traction. They are very similar in gameplay and style. RFOM feels a lot like CODIV to me for instance. They don't lack anything individually and are all excellent titles, they just don't seem to distinguish themselves from each other and the multiplatform games. Edit: The most distinguished title availabe on the PS3 is MGS4 when considered from the perspective of its similarity or lack there of to other titles available on the system
For the gamer, who wants to play the same game ad-nauseum over and over? Its great to have a quality title or two in your collection, but what incentive is there to buy KZ2 if you're already loving Resistances multiplayer? Even Sony understands this, and one of the reasons given for KZ2's delay was the fact it was "too similar to RFOM2".
The Xbox360 Exclusives - Left 4 Dead, Gears of War 1/2, Halo 3
The interesting thing here is : All of the above are unique between themselves and unique when compared to the multiplatform games available. They round out the multiplatform games without competing directly with them. They each have their own style and they have each got unique gameplay elements that set themselves aside and above their competition. They are also made by three of the BEST FPS develpers so they bring something special to the table. Valve, Epic and Bungie are quite an amazing combination.
For the style of games the PS3 has, multiplatforms are perfectly comparable. For the style of games the 360 has, the PS3 doesn't have such variety.
The quality is there individually yes for both platforms. The thing is this, Microsoft has just done a better job at bringing the gamer a better lineup to enjoy.
This is not a jibe on Sony, I haven't said they lacked quality FPS games. They are all excellent. What im saying to sum it up is that the Xbox360 has a better lineup when you think about all the games together as a package.