Assuming equal return rates, in theory (i.e., statistically) prioritizing the Roth will be better because the investments will have longer to grow and are not taxed. E.g. if you will get $1000 returns in one account and $500 in the other, you would prefer the $1000 to be in the Roth.
In practice, the vagaries of short term market moves can toss statistics on its ear. But you might as well play the odds, slight though they may be, and do the Roth first.