Videl rocks both but short hair is better. Admittedly, part of this is just because her fashion sense seemed to improve when she chopped off the pig-tails and it's kind of romantic that she did it for Gohan. But to justify my reason more, it fits her character better and I actually think makes her stand out more than the pig-tails. Like one of the main reasons Videl was so cool was because she was a fighter. Unlike every other woman in the series, except 18, Videl was a martial artist. Like Ranfan wasn't a serious fighter, she just fought with fanservice. Launch just fired a gun like she was America. Chi-Chi I guess did make it to the World Tournament proper, but she didn't seem a dedicated fighter, I think she just did that for Goku. And she never received the focus Videl did so it didn't matter. So what better way to visually identify this then by giving Videl a spikey haircut similar to the Saiyans. Which makes her fit in more with them, which is good because it serves as visual representation of her developing relationship with Gohan, but it makes her stand out more against the other woman characters in Dragon Ball, since until Caulifa, Videl was the only woman with that kind of hair style.