The tankless is more efficient and provides "endless" hot water (at a limited, but reasonable flow rate). But it's far more complicated than your current HW heater and will have issues from time to time. It's also important to get it serviced regularly where a conventional HW heater can be ignored, for the most part. Ideally you'd drain sediment from the bottom of the tank every few years and change out the sacrificial anode every 5 or 10, but you can often get away without doing either. Tankless has to be partly disassembled and cleaned every 1 to 5 years. Or just replace it after it fails every 7. It's more like a furnace or boiler than a water heater.
You may need to have the line to the water heater changed out for a larger one. If you currently have two power vent HW heaters, that's less likely to be an issue than in a lot of more typical installations. Atmospheric HW heaters are usually 35 to 40 Kbtus while powervent are often 50 or 60. Two power vent would be 120k btus. Tankless water heaters are usually 140 or 200k btus. Big jump from 40 to 200, not a big jump from 120 to 200 (so existing gas line is probably OK).