This is a plan-design comparison rather than a market comparison. HMOs and PPOs both exist inside and outside the Marketplace; what separates them is how you reach a doctor and what happens when you go outside the network.
An HMO routes care through a primary care physician who issues referrals and keeps everything inside a defined local network. A PPO lets you self-refer and usually pays something out of network. HMOs are generally cheaper. That discount is the referral requirement and the geography.