Both are important and I buy both. However, I will admit, the games I invest the most interest in are exclusive titles. For the PS3 so far I bought Sly: TiT and I intend to get GOW:A, The Last of Us, Beyond 2 Souls and Ni No Kuni. But my MP list is longer which includes DmC, MGR:R, Bioshock: Infinite, Tomb Raider, Castlevania: LoS2, GTA V, Rayman, Ninja Gaiden 3 RE, Dragon's Dogma (the updated edition), etc, etc etc.
With that said, I hardly discuss multiplat games, and some of the previous posters had good reasons why. Nothing against those games it is just more often than not exclusives tend to lead to better/more pertinent discussions, especially for the purpose of comparing things.