they are pretty much the same, its just that in PvP you can attack the horde (or alliance) in neutral or alliance territory. when you enter neutral or horde territory on a pvp server you automaticaly put PvP mode on wich allows horde to attack you. on PvE servers you have to type /PvP command to turn it on or attack some one thats allready pvp tagged. NPC alliance/horde guards can attack players without PvP tag
i prefer PvP servers at it feels more realistic and you chose right to not chose RP server as even if you like RP you wont get it as 60% of the players dont RP and theres a constant fight betwen the RP guys and the other players (what tha **** are they doing on a RP server is a question i never managed to find an answer to)
and yeah, one more thing, when you playing WoW dont forget to start the day with a good Chuck Norris joke