Absolutely not in your scenario. Give Delta your NW WorldPerks number. Note that you can't do this online at delta.com except by the "chat with a rep" function. You either have to call

Totally unrelated to the merger, for years you have been able to earn NW WP by flying on DL (or vice versa). That's because they are both partners in the SkyTeam alliance.

For the relatively low (sorry, but 20K is low by FT standards) amount of miles you have in NW, there is no benefit and lots of downside in "spreading it around".

IF DL and NW consummate their merger (my SWAG is 97% likely) then at some point, NW WorldPerks will become Delta Skymiles. But that could take a year after the merger. Meanwhile your 20,000 miles (I assume when you say "I've got over 20k in Worldperks" you mean "award miles" what we call RDM for Redeemable miles, not "status" miles) won't get you any free ticket on any airline. And even if you also had 5000 in Delta, the 20,000 and 5000 could NOT be combined for a free ticket - until the programs merge.

Whereas if you post your Delta flights to your NW account, you're that much closer to the 25K miles you need for a "free" ticket. Plus NW's "free" tickets are closer to actually free (lower fees) than DL's "free" tickets.

Likewise if you ARE talking about EQM status miles flown in 2007 (or MQM=Medallion Qualifying Miles) in DL's lingo), hw711's advice is dead-on-target. You absolutely will get NW Silver, which will become DL Silver when they merge the programs, if you earn 25K EQM on NW. There is absolutely no guarantee that you will get any elite status at the merger if you have earned 20K NW EQM + 5K DL MQM. Even if the "New Delta" does add up EQM/MQM earned in each program for 2008 flying to determine and go-forward grant 2009 status, that won't happen until the program merge, months after the airlines themselves merge. Whereas if you get 25K NW EQM this year, you'll have Silver status on NW that is also good on DL and all the other Skyteam partners from as soon as you earn it through 2/2010.

So whether you're talking RDM or EQM/MQM, put all your eggs in one basket for this flight and any others on Skyteam airlines. In your situation, that should be NW even if you're flying on DL, CO (still in Skyteam through at least mid-2009), or any of their other 10 or so partners in Skyteam.

Oh - and don't get confused by my newly updated profile next to this post showing I've replaced my NW Silver status with DL Gold. That's because I just got Delta to match the higher-level status I have with United. That puts me in better shape with the new DL/NW for 2009 than I would have been with continuing to earn in NW - but that's my situation, where I had a non-partner higher status to show to DL for a match. Unless you have a similar status (with AA or UA or US - not NW or CO), then you should keep using NW this year and next until NW becomes DL.