Northwest is offering a 100% bonus on miles transferred between accounts through May 31. The annual maximum miles transferred into an account is 30,000 at a cost of $325, and yields 60,000 Northwest Worldperks miles in that account.

Naturally, Delta Skymiles is making the same offer, 30,000 miles transferred costs $330 (their transfer fee is higher, oddly enough) and yields 60,000 Delta Skymiles. The annual mileage caps are much higher than with Worldperks, 300,000 miles into any account and 150,000 out.

Of course, Northwest and Delta miles can be transferred back and forth between each other at will which means you can use Delta’s higher transfer limits to circumvent Northwest’s transfer cap.