Just in case there's a terminology question, an HMO is a Health Maintenance Organization. It requires patients to select a PCP (Primary Care Physician) who is used as a "gatekeeper". Referrals to specialists (except yearly eye and gyn exams usually) require pre-authorization from the PCP. There is a network of physicians who accept a given plan and all medical visits must be to physicians in the network. This isn't such a big problem in large cities (such as Boston) because the vast majority of physicians are in the network of the major health plans. However, in outlying areas, it can be difficult to find doctors (especially specialists) who take the plan. Referrals to specialists who are not in the area can be considered "in network" if the treatment is not available in what is typically considered the network (eg, transplant services for patients in rural areas may require referrals to major cities).
PPOs, Preferred Provider Organizations, function much like HMOs except that patients are free to use out of network care, albeit usually with a higher copayment. HMOs don't cover out of network care at all (except in cases of emergency). Patients do not need referrals from their PCP to see a specialist.
As a physician, it shouldn't matter what kind of insurance your patient has. If you are a PCP, PPO patients can seek care wherever and whenever they want, so it's harder to coordinate care. But overall, they function similarly and often have similar billing rates. PPOs open up a wider network of physicians (because of allowing out of network use), but therefore typically have a smaller network than an HMO.