Do I have to pick only one? Because it VERY much depends on the game, context, and my involvement in the game.
Single-player, narrative-driven game? Probably 'stack it'. I'd rather a meaty 'expansion pack' type of DLC that adds a big chunk of story and gameplay. Whether I think it should be free or paid depends on the content of the 'expansion'.
That said, I'm not beyond a small trickle of extra content being added to my single-player games. As long as I'm not having to buy it every time, I don't see the issue.
Multi-player game? Pad it. I usually point to Dota 2 as my example. I like my core game to be hefty, with it receiving constant content updates over time to extend its life and switch up the meta. And I prefer gameplay content to be free with MT content being primarily cosmetic. (with a few exceptions)
Generally speaking I am NOT a fan of Season Passes, on principle and from experience. I don't like throwing down a fixed (and often hefty) chunk of change for the promise of 'forthcoming content that's totally going to be worth it once we figure out what it is'. Fuck that.