I think that if you care about slowly growing up your account then you should play darkorbit because private servers usually have a short life span, you may not even build a good account and they will close the server, and in pvp you can enjoy what the essence of this game is play with other peoples, any game in which you can face the second living person is 100 times more interesting then just killing npc.
And not that I'm defending Oneultimate I play also on PinkGalaxy (pvp) server admittedly it is not always online it has fewer players and works slowly but we have havoc/hercules and all this new stuff.