I agree with the other posters, who are all at the FF guru level. Mileage pooling and miles/segments counting towards elite on other airline have pros and cons, whichever way they are implemented someone will complain about it. I like the way it has turned out with United and Delta. Regardless of whether or not United and Delta are partners, it makes sense if one can to concentrate on one airline (if one can) so that one reaches the highest elite status on that airline. I have been happy with United and do not know much about Delta's elite program, I suspect that they are similar. See which airline is more convenient on the routes you fly, do they serve your destinations, convenient schedules, plentiful flights etc.