I have used both a focus and a shield and i say shield.... When i'm running away to go stop a ball run, stop a cap etc.. if im being autoattacked in the back i see the shield cut that dmg way down.. Yes it wont stop everything, but i believe the survivablity it gives me is better than the extra force power. Honestly if you try both for a few games you'll notice that 1v1s, 2v1s etc are easier with the shield.
23/1/17 is seriously monstrous in PVP. I've won 2v1's against some of the top rated pvp guilds. I have a smash sentinel, vanguard, sage, sorc, and my shadow honestly is the most op of winning the game. Yes my vanguard/Sentienl can do a ton of dmg.... but the control i have with my shadow being able to do dmg, survive, stealth, sleep, sprint... its a wonderful class to play and it takes time to master.
I recommend to eveyrone to roll a shadow/sin
Also just carry around 2 sets of full war hero in your bag, stims, and field respec.