I agree with Kacee above that it's quite difficult to focus exclusively on DL out of SFO. I've been moving some flights away from UA for reasons discussed elsewhere, but I generally only wind up on DL when I am willing to connect and/or when I am elsewhere in the country.

Generically, the only possible reason I could see for sticking with them is if you're certain you can hit Diamond and if it's sure that a status match DL to UA would then get you to 1K on UA (I don't have experience with that), all caveats for 1K value as noted by others above. Otherwise it seems you don't have anything to lose just matching now.

Do you have some regular travel patterns that you can use to game out possible itineraries? That could influence the calculus.