Firefight has more teamwork and isn't ruined by other people knowing the mission. And the firefight voices made it a lot more fun.
My experience with Spartan Ops is that if you don't get into ALL of the missions the day they release (and sometimes even if you do) then the majority of your team will just rush the objectives and kill whatever story progression or level pacing there is. And they're always the same scripted experience, so replaying them to catch up on that story and pacing isn't fun.
I liked firefight better because you could screw with your friends as they cry when near invincible hunters bash them to death, while you slip around behind them cloaked as an Elite to assassinate them.
I would choose Spartan Ops IF the option to choose any of the chapters for multiplayer was available instead of just the Episode of the Week. The first time through for the story and immersion. After that I find the pacing and goals more enjoyable then just wave after wave of mobs with no objective.
But because Firefight doesn't limit you like Spartan Ops does, I'm going with a tie.
By the edge of those wilds, warriors have charged. But now the years have passed, the generals have left and the grass grows in the armors hollow on the hill.