Rex has three possible routes. You decide which route best serves his desire to remain employed here:
1. The Sanchez Option: He can start Sanchez in the hopes that Marty Mornhinweg is The Guy who gets Sanchez to play like Jeff Garcia.
REX PERSPECTIVE: Getting Sanchez straightened out will go over well with Woody, who will finally get some return on the $50 mil he's paid Mark Sanchez. The risk is that Sanchez sucks, the team quits because they're sick of Rex's love affair with him, and Rex gets whacked at mid season.
2. The Garrard Option: Garrard's a veteran who has been relatively successful in the league. If he can be a net positive at the position and not turn it over, Rex's defense could conceivably carry the team into playoff contention.
REX PERSPECTIVE: Winning with what is projected to be a bad team will boost Rex's profile instantly, and will get him extended here, or an immediate hire elsewhere. Benching Sanchez could also get Rex a clean slate in the locker room, showing that he's no longer coddling a loser. The risk is that he burns bridges with Sanchez, Garrard gets injured, Geno's not ready, and the season implodes.
3. The Geno Option: Geno could be Russell Wilson.
REX PERSPECTIVE: Playing the rookie early instantly drops expectations for job performance into the basement, and makes the likely 59-3 drubbings by the Pats and Falcons more understandable. Also gives Rex the chance to look like he's part of the rebuild, as opposed to being the reason for the rebuild. The risk is that Geno can't overcome the lack of offensive talent and melts down, and the season implodes.
This is a pretty good analysis - the one thing you left out for Sanchez is the fan reaction. If Sanchez is the QB this year he can't just be "better", he has to be almost perfect. Sanchez is going to be booed relentlessly this season and if he doesn't start 'hot' there is really no way I can see Woody tolerating a season of fulltime booing.