While I generally lean toward sewing most of the time, in this case I'd say paint unless you already have experience sewing spandex-y stuff.
1. the letter is broad, covering a lot of the torso, giving you lots of places to go awry.
2. you ordered a suit. This means trying to sew on a finished garment instead of the freedom of working on one piece or layer without being encumbered by the rest of the garment. Plus a lot of morph suits are really thin spandex, making bad stitches potential holes.
3. the letter itself is very narrow, meaning skinny placement and tight sewing corners and turns.
I suggest making a cardboard template of your torso to stuff into the suit in order to get good stretch while painting. (it also keeps the paint from bleeding through to the back). Paint the WHITE parts first. They probably will need two coats as most white fabric paints are not as opaque as most like. Then do the black, which will cover any wonky edges of your white paint and give you nice crisp lines.