This is how I understand the differences:
A direct vent uses either a concentric pipe or dual pipes to get external air and vent the combustion gasses. It needs to be closer to the exterior than a power vent.
A power vent uses a fan to push the exhaust gasses further than a direct vent. It gets combustion air from the inside, so it relies on air leaks to enable the burner to run.
Given a choice, I'd go with a direct vented unit - quieter in operation because it doesn't have the fan which can wear out and require replacement, plus, it can be noisy especially when it ages. Nasty if it's near a bedroom window.