Good topic.
It's a tough call. The logical choice would seem to be Yao. It's rare to find a player his size who can do all the things that he can. He has the tools to become a great centre in the NBA. But it's well documented that he's selfless to a flaw. The phrase "gentle giant" is starting to be used more frequently. Obviously, no one wants another arrogant, selfish star who hurts his team more than helping it win. But Yao is indeed a potential 20-10 player (with 2-3 blocks and a couple of assists thrown in for good measure) if he develops the right mentality...and gets the opportunity.
Despite reasonable numbers in the assist column, Francis still isn't a pass-first point guard, and Cuttino Mobley is definitely not a distributor either. Trading them away would definitely allow Yao to be the focus of the team's offense, and the foundation that the team would be built upon. But what if Yao can't be a guy who scores 20+ ppg? If nothing else, Francis and Mobley aren't shy about putting points on the board.
I believe that every team needs a player who is willing to put up shots no matter whether he starts out by hitting his first five attempts or missing his first five attempts. One of the reasons the Timberwolves crumbled in the first round last year was the lack of such a player. Sure, let's give the Lakers credit where credit is due, but the Wolves' second best scorer, Wally Szczerbiak, attempted fewer than 10 shots in half of the games in the 6 game first round series.
Now consider the Timberwolves this season. One of the top teams in the East, with three players willing to take a lot of shots. There are times when selfishness (or what seems like selfishness) is an asset, not a flaw. Teams need that guy who isn't afraid to put up shots when the team needs them most.
This is not to be confused with players who simply jack up a lot of shots to score a lot of points so that they'll look good as an individual. I'm referring to players who can score pretty much whenever they're called upon. You know the type, so I won't bother naming names. So the question is, are Francis and Mobley (particularly Francis) those kinds of players? If not, can they be?
If all three can co-exist, then the Rockets are a force to be reckoned with. If everyone is fine with Yao being the main man while Francis swings back and forth between distributor and scorer (similar to Stephon Marbury) and Mobley continues to score whenever he gets a chance, then it's simply a matter of finding the role players to play alongside them.
But it seems like that won't work. So, what do to?
The Rockets have already tried Francis and Mobley for a few years pre-Yao, without so much as a playoff appearance. They've become better over time, but it still hasn't worked out very well. Getting rid of Yao and keeping a guard-orientated offense with Francis as the centrepiece probably isn't going to work much better than it did before. But going with Yao depends on his willingness to take a lot of shots and put the team on his back.
It's very difficult to make a choice between the two.
So, I think the Rockets should give the Yao-Francis tandem another go. However, Mobley might have to be shown the door. If Yao needs more shots, they have to come from somewhere. Mobley should not be unattractive to other teams, and probably won't be happy in Houston if he's not getting as many chances to score. The Rockets will be able to continue to mold Yao into a dominant force, while having the security of a proven scorer in Stevie Franchise.
If that doesn't work...figure out which one of them would work out best as the lone star, and trade the other one for a sidekick.