To join Delta just to get 1000 miles after your first $25 dine is not as good as it sounds, as you will only be earning 3x miles instead of 10x (5+5) miles with Northwest (Assuming you are already VIP status with Northwest and rate your dines).

If you have a $25 dine, you will earn 75 Delta miles or 250 Northwest miles, with the (250 mile) bonus you are +75 miles.

If you have a $35 dine, you will earn 105 Delta miles or 350 Northwest miles, with the (250 mile) bonus you are +5 miles.

If you have a $36 dine, you will earn 110 Delta miles or 360 Northwest miles, with the (250 mile) bonus you are even in miles.

If you have a $37 dine, you will earn 115 Delta miles or 370 Northwest miles, with the (250 mile) bonus you are -5 miles.

Any dine higher than $37, you lose more miles.

Any future bonuses you try for, you will still be at 3x miles with Delta. If they stop the Rate your dine bouns, then it becomes more favorable (3x vs 5x) instead of 3x vs 10x.