I know the usual rule is to concentrate one's flights on one airline to maximize benefits. Generally, I agree. Is this an exception?

I switched from Delta to American in mid-98 and am Delta Gold Medallion for 1999 based on early 1998 flights. As long as I am at least Silver Medallion I can use 200,000+ previous program miles for awards under Delta's more generous previous program Medallion award schedule. Otherwise, I have to claim award certificates by the end of this year (fly any time during 2000) or use the miles under the current award schedule.

Situation: I'll requalify for AA Platinum AAdvantage with my next flight. I have zero chance of making Executive Platinum because I usually fly discount coach so I'd need nearly 200,000 flight miles. However, if I switch to Delta after my next flight I have a good shot at 25,000 DL miles before the end of 1999. That would keep my Medallion string going one more year. It would also extend the life of my DL miles for another year, but that's a minor consideration because I'd probably fly with them again anyhow before the miles expire in August 2001.

The question: other than four 500-mile AA upgrade credits for every additional 10,000 miles with them, what - if anything - would I give up? Assume AA and DL miles are equally valuable. (I have comparable elite status on both airlines, so both will give me double mileage for all flights.)

The combined wisdom of FlyerTalkers exceeds that of any other known group of humans. I appreciate all comments and suggestions.