Depends how you fly and how you upgrade.

American may have better upgradeability on more fares? but Delta has less points required to get an upgrade (only 10K)--are select fares same as American's any fare?

I go with the airline that gives me the most perqs (ie. easy upgradeability) and then the one that gives me the most miles. Everyone has to decide for themselves on this one. You said that you would likely use the Delta miles by 2001 anyway.

I would go ahead and get your Delta rewards now and continue to fly AA (more partners, oneworld etc.), if you like their service and if they can regularly upgrade you and if getting reward travel is relatively easy.