Actually, HMOs and PPOs can have the same level of coverage. PPOs are sometimes cheaper than HMOs, sometimes its the other way around. At my work, there are two PPOs and one HMO offered. The HMO is the most expensive of the three plans and doesn't offer much more than the better PPO.
HMOs require referals for any speciality services, PPOs do not. For that reason, I always pick PPO when its offered. Saves time, if you know what kind of doctor you need.