The Long-Haired Emi looks noticeably more mature than the other ones. I think that this is because of the different eye-shape she uses, which is usually used more with adults. Combine that and the aura of confidence from the picture and you get a highly attractive Emi.
i love forehead showing, i always love that look, it's cute. short is also cute long hair...is kinda sexy IMO. not a huge fan of sidetail but w/e ponytail is pretty and good ol' original twintail version is always good!