It may be better for Mills to play, but the team needs a pick-me-up that a win could provide. If Taylor is 100%, the consensus seems to be that he's the better chance of providing this win. Plus, the Texans play the Titans following the bye.
The medical reason Taylor didn't play last week is that it was his first week to return to practice and his hamstring being stressed. The Fins are an opponent that won't provide a lot of pressure. The perfect opponent to take his return to game conditions to the next step. With the bye, Taylor should be ready for the Titans.
Chances are good that Taylor will reinjure his hamstring and Mills returning to starting. Sitting a couple of games won't do Mills any harm.
And there's the traditional adage, though not always followed, that veteran starters don't lose their job because of injury.
And finally, I'd personally like to see Taylor's performance with the Texans current starters, just for comparison. All part of Mills' evaluation process.