Can't say I'd prefer running Lacy all day because he doesn't have the upside that Rawls does, but Rawls has committed waaaaaaay too many mistakes over the last two games to get the nod as starter.
Off the top of my head....
The Giants game included a fumble, dropped screen pass, outrunning his blockers on the other screen pass, missing some reads. Texans game consisted of penalties negating a TD, not hauling in a catchable pass, running into the backs of his linemen, missing cutbacks for easy yards.
Lacy is and always has been a downhill runner in the NFL. He's not going to create much on his own, but give him a hole he can run through and his momentum is going to get yards no matter what. His vision so far has been slightly better than Rawls from what I've seen, he just can't get anything going if he's being met at the LOS or behind it.