All that I'm going to say is premised on NW still being a viable business. If they resolve their problems with the FA's, then they can probably still stay in business. In which case, I think that if your primary FF goal is earning RDMs for future trips, NW WorldPerks is a better program than either CO OnePass or DL Skymiles.

Two major reasons:
1) If you're flying the "home" airline, in this case NW, you're more likely to have to connect no matter where you're going, unless you're going to MSP, DTW, or MEM. That means more miles from the connecting flights, as long as you avoid the dreaded "direct" flights.

2) Promos. Promos for extra RDM earnings seem far more generous, especially if you make use of partner non-air earnings such as hotels, cars, online shopping. NW typically runs two or more big promos a year where either by explicit miles bonuses (e.g. the just-ended Bonus Play promo) or by points-based promos (e.g. the current MilesSafari, or early this year's 100,000 Mile Dash, or last year's Extreme Miles). I've gotten tens of thousands of RDMs from these promos even though I only started working NW WP serious barely a year ago.

Also it seems that NW has at least as good low-level (PerkSaver) availability than does OnePass or SkyMiles.

But if NW fails, where its miles go if anywhere is a crapshoot. So I'm in dual earn and burn mode, where I don't hold onto their miles for all that long before redeeming and flying. If you assume NW will stick around, or its program miles will be accquired the same as AA did TW or DL did PA years ago, then it's worth it.

Forgetting the NW survives/fails dimension, there are some reasons why CO might be better.

1. Unless you are crediting miles to OnePass, you have no way to get full EQMs on deep discount fares flown on CO. Since you're in NYC, CO may be the most convenient to fly, but you'll only get 50% EQMs on CO in NW's or DL's programs, even if you purchase on continental.com. If you purchase at CO online and credit to OnePass, you get full EQMs on all fares. Given your travel patterns just barely bring you to low-tier elite level, you might need these to maintain elite status.

2. Upgrades might not be your top priority, but they're nice to have. If you'll be flying CO a lot, you're more likely to get an upgrade on CO as a CO elite at a given level than as an NW elite. I believe that CO Silvers get upgraded ahead of NW Silvers on CO. Reverse is true of course if you're mostly flying NW.

Since you say you currently fly DL most, then maybe DL is your best bet. You'll get the chance at upgrades (but only on DL, no reciprocity with CO/NW). You also won't have the agony of trying to get the DL call center to properly input a different Skyteam FF# and assign elite area seats.

But I think you'll earn the most RDMs in NW if you use the promos bigtime.

I know, no single answer, sorry. But hopefully the pros/cons help your decision.