I guess it depends! If you're going for accuracy, you should pick whatever you can pull off best. If not, just pick whatever would be most convenient for you!

If I were going for accuracy, I'd pick Yuna since I look like her more than I do Lightning, but I definitely like Lightning's outfit better. It's easier to move around in, has pockets, and it's much more slimming! Not to mention Yuna's outfit may show a little more side boob than I'm comfortable with.