I guess it depends on the promos and how you use them. If you actually fly one of them a lot, perhaps NW, as the promos involve a lot of flying to pay off. But if you're not flying them that much (and it's hard to from LAX, where the OP is), I'd say Delta may have better promos, because they have way more promos that you can do on the ground based in LAX. For example, right now, sign up for DL SkyMiles Dining (their flavor of iDine/Rewards Network), and do 5 $15 minimum dines in the SoCal region by the end of August, and you earn 5000 bonus miles. (And that's their second dining promo this summer!) Also, I signed up for Delta's program in June based on a promo with Starwood, and got 4000 bonus miles just for signing up that way. (And I wasn't evern SPG before that; I just read about that promo on FT, signed up for SPG, and a minute later, once I had my SPG number, signed up for Delta.) And then Amex offered me a Delta credit card with 10k bonus miles with one purchase (tho I made 5 purchases, those 5 dines ), and no annual fee the first year. And then I earned 1000 bonus miles for a couple two-day Avis rentals when no one else would have given me close that.

I've been with NW for several years, OTOH (tho just as a partner collecting airline, only actually flew NW LAX-MSP-LAX once and hated the seats horribly), and have seen no dining bonuses for quite a while, and without flying them (and with them as neither my primary nor seconary collecting ailrline) can make limited use of most of their partner bonuses, while Deltas have lately been so bountiful for so little spend that I can't pass half of them up even though they're no closer to a "primary" collecting airline for me than NW is.