I just don't get excited by Farrell at 12, never have tbh. Maybe I just like a hard carrier to the line but for all he can kick/distribute better than the average 12 I don't think it replaces the battering ram option or a pacey runner with a step.
For me it depends how much influence Townsend is getting in the selection, Gats wanted him on the last tour and now this one so he rates Gregor's style. I would think with a 10 of Biggar's current form and ability he has to be the shoe in, I think he can pull off the moves which makes Gregor's attack dangerous. I think Finn will be half in half out, likely a bench option because his way of play can change things rapidly for the last 20. As a Scot I would love to see him develop some more stability to his game, could be a top choice for the next tour.
The centres I am not so sure, if Hogg is played at 15 he is such a threat in attack he drops in and out of the centre position to suit and would give an edge that may complement an Aki/Henshaw or Aki/Daly lineup (who else is the "real" 12 option?).