From the executive summary:

In this study, researchers compared the effect that having a choice of health plans has on California Medicaid beneficiaries enrolled in managed care. Beneficiaries without a choice of plan enrolled more quickly, while those with a choice were more likely to delay enrollment in a plan. The authors also found that beneficiaries with a plan choice were admitted to the hospital at a higher rate for conditions that could have been treated earlier in an ambulatory care setting.

Full Report: Unhealthy Competition: Consequences of Health Plan Choice in California Medicaid (PDF | 158K)

The Commonwealth Fund. (2010). Unhealthy competition: consequences of health plan choice in California Medicaid. Millett, C., Chattopadhyay, A., and Bindman, A.