As it happens, I was riding alongside both of those exact bikes at the weekend. They belong to mates.
They both get on very well with them, the Spitty owner coming from a Rune and that was his first time riding the Spitfire. He said that the Spitfire had a little more noticable feedback through the pedals when it got rough compared to his previous 160mm Rune. I guess that makes sense, but that it climbed very well, and was still excellent on the rough downs of Eastridge.
I preferred the Banshee myself as I kinda want one(I currently have a 2012 Mega)
The other mate on the newer Mega gets on very well with it seemingly. Can’t ever keep with him on the climbs but that’s probably more to do with fitness etc than bike.
Both great bikes though and I reckon you’d not do wrong on either.
Personally though, my worry was that the 140mm Spitfire may feel a little out of its depth on some of the rough Alpine DH tracks. My 2012 Mega already gets a bit bogged down abroad and on uplift days at some of the classic UK dh tracks.
Everything is a compromise though and the Spitfire is a fantastic frame. Mega is too though…
Glad I could help… 😉