I'd always recommend going with the best single card solution you can afford. In the past I've had 3x580, 2x680, 2xTitan, 3xTitan Maxwell, etc. There was a time around Crysis 2 when SLI support was getting close to a certainty in main titles on launch day.
But in recent years - and especially around DirectX 12 - this has changed a lot. Even it when it works there are issues around flickering textures, shadows, frame pacing and so on. None of the recent demanding games I've played (Atlas, Metro Exodus with RTX, Ace 7 with 150% scaling, etc.) have supported SLI. Division 2 beta also didn't support SLI, to mention another recent example.
And often when a decent SLI profile does arrive, I've finished the game..
I think the only real exception to all this would be if your favourite title has a good SLI profile (especially if it's a multiplayer game around whose communities you spend your evenings). I'd recommend checking that game's forums and if it's a demanding title and there is a working profile, that changes things.