Twilight : "Ya rainbow ankle-biter!" "That'll slow yer down, ya twitchy hooligan!" "Ya miniature delinquent! "Couldn't skip around that could ya, ya precious little pony!" "Ya spastic little gremlin!"
Rainbow : "Snipe dat, ya frickin' coward!" "That fancy scope of yours, bet ya got a REAL good view of me killin' ya!" "You looked da whole time for dis?!" "It was a mercy killin', ya live in a... air balloon!" "You'll never hit me! You'll never hit my tiny head! It's so tiny! I got a frickin'... such a tiny li'l head!"
Kind of late but Id put my money on Rainbow dash other source or not her power still is coming from her element of loyalty. Id imagine she is fighting to bring Twilight to her senses for this reason her loyalty is burning through to her core. If you remember from sonic rainboom when her friends are in trouble she can bypass her limits. So in the end Twilight stands no chance as Rainbow by default is the same pantheon of Simon the digger always able to pull just little bit more power through shear force of will. example: [link] though it might be a draw like this: [link]