Wasn't the PS3 famously faster than the Xbox 360? Though I suppose that's hardware, what software are you referring to?
It comes down to games for me, exclusive titles make the difference. When I had my original Xbox, I was jealous the PS2 had GTA and I didn't, but those days are long past. Now with Halo and Gears of War on the Xbox, I don't know of anything Sony has that I'm dying for. Diablo 3? I already have it on PC, so that wouldn't sway me.
The PS3 cell CPU was a little faster and also helped there slow GPU. I own both consoles and could never see a noticable difference. MS is a software company and has always had great software for almost everything. Sony lacked good development software for the PS3 making it very hard and more costly to design games for.