The main difference is their venting systems—the former requiring one and the latter not. This is due to their differences in heat production.
While vented gas fireplaces use outdoor air to provide combustion and burn at a lower temperature to create a realistic flame, ventless gas fireplaces use air from inside the home to provide combustion and burn at a higher temperature to ensure no gases are leftover.
Although ventless options are more efficient, they aren’t designed to be the sole source of heat for a property. Therefore, their difference in home heating function is usually the largest deciding factor.