I see what you're saying about Farrell, but I don't think he's led his team better than Sexton though. It's not because England are much better on attack this year compared to last year that it's down to Farrell's conducting. He's been playing the same style, although he's improved. It's the change in the makeup of the England backline that's allowed a lot more activity and impact on games I believe, not so much Farrell the initiator of that.
Like I said I like Farrell and I like what he brings to the field but Sexton is just more mature in his game at this point and I don't think England attacking better can be used as an argument within a Farrell vs Sexton discussion. Sexton, aside from his other qualities, leads a much more complex set of schemes on attack in this Ireland squad, a more demanding role which he fulfills; I don't think if anything one could argue against Sexton in Farrell's favor in that department.