[PvP] Help me - Shadow or Rogue

I've decided to reroll to a new class. Mainly I'll be doing arenas on Lordaeron. Atm I got a lvl 37 spriest and 41 rogue and I can't decide which class to pick.

Now for the priest, I've decided I want to play shadow only, going with this spec: http://wotlk.openwow.com/talent#bxIr...0xfiRfkbfofkAo
Something like this.
Now the thing is, I got tons of experience only as a melee. Fake casts and stuff are new for me.

As for rogue, I've decided to go mutilate: http://wotlk.openwow.com/talent#f0ec...0uZ0bZebfbc0db
Like this. Focusing on Envenom vs platers as my main damage dealer.
Now with a rogue I can get more creative, which sounds kinda fun. Also got the experience with melees.

I'll be rolling a Dpr/Rogue or a Spr/Rogue as a Rogue.
While as a priest I'd go Spr/Rog, Spr/Mage.

So which class is more fun? Which is better in 1vs1? Which is more viable for arenas? Which class needs more PvE gear?

Both are fun and viable in arenas, even both can do great in 1v1. It's all up to you, really, no one can say what you should play. Rogue however does need more PVE gear (offparts).

Spriest>rog

The answer, as always, is go on blackrock and make a rogue and a spriest and play them both. Do a bunch of bgs, duel a few people, do some arenas and then decide what you want to play.