I look at it like EJ is the better overall quarterback while Cassel is the more reliable. As a starter I much prefer EJ, but the backup job isn't about upside to me as much as it is limiting downside. Cassel won't win on his own but his style and strengths lend themselves to giving the rest of the team the chance to elevate their game and find a way to win. EJ is capable of astonishing things but you don't know what you're getting on a given day- he could give you zero chance to win like he did in Houston.
As others have said, if Tyrod is out multiple weeks I much prefer EJ, he keeps the whole playbook open and unlike Cassel has a shot to be really good. If it's an in-game replacement I prefer Cassel, you know the ball will come out on time and accurately and you'll still have a chance to win that day.