Final answer:
A PPO offers more flexibility than an HMO by not requiring a primary care physician or referrals for specialists, and it allows you to see doctors outside of the network at a higher cost.
Step-by-step explanation:
When comparing a Health Maintenance Organization (HMO) to a Preferred Provider Organization (PPO), the primary difference is that a PPO offers more flexibility in choosing healthcare providers. PPO plans do not require you to select a primary care physician (PCP), and you do not need a referral to see a specialist. In contrast, HMO plans typically require you to select a PCP and get referrals to see specialists. Moreover, with a PPO, you can see doctors outside of your network, though at a higher cost, while HMOs often limit coverage to providers within their network unless there's an emergency.