Yeah. You'll just have to try both to see which you like better. If you've already written the tune in D major, A mixolydian isn't going to change the tune any. So if you don't want to change the tune, there's your answer. Although theres nothing wrong with experimenting and changing the tune from D major to D mixolydian might be better. Try it and see. :) If you haven't written the tune yet, try both to see which one you like better.