Originally Posted by intuition

It depends on your own preferences.
If you only care about retaining status with A3, ie you only care about collecting tier miles, then it does not make sense to accumulate more miles to A3 until your new membership year starts. This is because there is no roll-over of excess tier miles.

But in many other cases it makes sense to collect miles to only one program. This is because you will otherwise end up with 2 small stacks of award miles and it will be more difficult to redeem. Collecting all award miles to one FFP helps redeeming, even if some tier miles are lost.