Between NW WorldPerks and CO OnePass, I have found out a major advantage in the former.

All qualifying flights on many partner airlines (including Alitalia) go towards elite qualification, very much like the OneWorld network. On OnePass, all flights have to be either CO, NW or codeshares with CO flight numbers. I project to fly a lot on Alitalia, so I must favour WP over OP.

Award travel and elite benefits are equivalent for both programs. Finally, NW has a more extensive network to Europe and Asia, compared to CO.

Thanks for all your input.
Maple