I have been rocking the first spec that you have listed, and must say I LOVE it far more than anything arsenal. I grew incredibly bored with tracer missile spam and gave pyro a try. I love that I can remain incredibly mobile, kite and do massive amounts of damage. I will be honest, I hate merc survivability but I suppose we cannot have it all. Our damage output is so great that it makes up for it. On top of it, being as mobile as pyro is, done and done. Not sure I will go back to arsenal again.
I easily have the most damage output in almost all of wz's and that is basically what a merc should be doing. Pair the damage with knock-backs, the occasional heal and that is what you should be doing in warzones.