I could ofc play them both, but would rather focus on one and learn to play that toon to the max.
Priest is still my number 1, but the upcoming rogue buffs is ofc something im thinking about..
Just play the one you like the most. Unlike rogues, priests have always been kind of an "ok" class for arenas and rbgs, while rogues were a great class. But with the recent changes, even if our damage gets a nerf we will still be used as one of the best support classes (life swap, fears, heals, grip, etc), so pick the one you enjoy the most.
I mained a rogue from open beta in 2004 up through this expac, but am now playing a warrior.
In random BGs, rogues are easier to dispose of than priests (dispose = either kill or make them vanish and run away). I've played a decent amount of max level pvp on my priest, but I'm much more experienced with my rogue.
When I'm playing on my spriest, most melee makes me unhappy. When I'm on my rogue, as long as I save vanish for escaping, I'm usually happy fighting anyone that doesn't gcd-kerplow! me because I can get away.
That's all a round about way of saying that spriest damage and utility is awesome, and when you're not being trained, it feels awesome. Once you get trained though, life starts to suck. On a rogue, you don't have as much damage throughput or killing potential (ime at least), but you don't have to have your day ruined by that one warrior/dk/whatever that decides that you're going to be his punching bag.