For most people this comes down to premium against network freedom. For a veteran it depends on something else first: whether the plan is your main coverage or a backstop behind the VA.
If the VA handles most of your care
The plan is a safety net rather than your day to day coverage. You are buying it for emergencies, urgent care and the occasional civilian appointment. In that case the lowest premium usually wins, which more often means an HMO. Paying extra every month for out-of-network freedom you will rarely use is money spent on an option you do not exercise.
If you travel or split the year between states
VA coverage does not follow you, and neither does an HMO network. This is the situation a PPO exists for. If you winter somewhere else, spend months near family in another state, or drive long distances regularly, the out-of-network coverage stops being a luxury and starts being the reason you bought the plan.
If your nearest VA medical center is a long drive
Then the plan is effectively your primary coverage whatever you intended, because you will use civilian care for anything that is not worth the journey. Network depth matters more than premium here. Check your actual local doctors and hospital by name before you choose, and weight that above the monthly cost.
One more route worth knowing: if you qualify for Medicaid as well as Medicare, a Special Needs Plan may be available to you, and those are built around care coordination rather than network breadth. For the general differences between the plan types, see our guide to Medicare Advantage PPO plans.