Upside of having two characters?

[Fendred]

Having both male and female exclusive gear.

This. I will never have the lightning hair cut unlocked or lightning's outfit. Also, cascadier outfit can only be for one sex. To be honest though, male/female exclusive gear doesn't make a whole lot of sense when two people putting on the same kind of armor can have different designs. (What ever happened to the part of the dragoon armor covering the belly on the female model?! X.X).

However, if you intend to do the end game rep grinds or the atma -> animus -> novus questline, you'd be better off not making a second character. The time requirements on those things, especially when taken as a whole, are brutal.