well. in pve you can rpg and pvp.
in pvp u can just pvp. hehe
you unlock things from pve to use in pvp.
so. unless your gonna go play pvp mode often. it would be better to later create a pvp character, when youve unlocked evrything you want for your pvp build.
added: a 3rd and/or 4th character slot is also a good extra storage space later on ingame. (if your a person who likes to "collect" things. )