I tend to think Siemien is better than Tyrod. He comparatively, did more for his team to win games than Tyrod did, as a passer. Keep in mind he's young, had an awful run game, and had a much inferior O-line to Tyrod.
Fwiw, Siemien had games where he put up chunks of yards and you really can't forget that Tyrod has been anchored by Shady for two years. We like to say that Tyrod boosts the run game but it goes both ways and I'd tend to lean the run game is carrying more weight.