Can I go with both? I like certain aspects of each more than the other.
For Mio I prefer her shoulders and chest to be covered like in the original, but for everything else I prefer her Remake design. The multi layered skirt and ribbon on the back of her shirt are far more interesting to look at and they really went the extra mile with the added details to her leggings and socks (Although the latter won't be seen very much).
For Mayu I like both a lot honestly. The brown dress with the little lace at the bottom and long socks is really adorable, but the more intricate skirt and blouse design from the remake is much more to my liking. If I had to pick one thing I wish they'd kept from the original outfit it would have to be Mayu's black collar. The patterns on it as it frames around her neck during the Crimson Butterfly ending as she emerges from her cocoon (So to speak) looks really nice and I prefer her throat not to be showing as much.
For both of them I prefer them having black hair and having a darker eye color. The lack of light reflecting off of their eyes while not hideous is an odd decision when they have light in their eyes in pre rendered cutscenes, and the ghosts have light in their eyes in the remake as well. The brown hair can look really washed out and unnatural if you're playing it on an less than ideal calibration which in the case of my TV I was. It can be very distracting when you go from their old designs to their new ones because of how light their hair color can be at times.
I do like the way their hair flutters inwards and outwards compared to their original designs, and I love that their blouses are not longer brown but rather crimson colored. The butterfly designs on Mio's leggings and socks while stretching the realm of suspension of disbelief (She really wore all those butterfly accessories in a plot about butterflies?) look really nice and it's a great graphical detail seeing them reflect off the light whenever it shines onto them. Mayu's new butterfly wing sleeves are really nice and are more believable for me than the leggings.
Overall I'd have to say I prefer their DCB designs. They feel more like outfits real people would be wearing than the original designs and are certainly way more believable than Ruka's ballroom dress and panty hose, or Misaki's silk blouse nightdress combination.