The tankless heater will be much more efficient, but costly.  They are expensive to purchase and more so to install.  You need an outside wall and will breach the weather tightness of your house by installing the vent.  If you purchase an effiecient water heater tank type and you use a lot of hot water I believe you will come out ahead.  The other side of that is if there's just you and one other person that takes short showere and very little washing you could pay off the tankless costs in about three years.