An 80 gal electric with a 4.5 KW element takes 4 hours to heat up. It is capable of suppling 10 gal. per minute for 8 minutes. This enough to fill a tube in a couple minutes. The difference in efficiency is 10% for the standby losses.
The complexity of the indirect is significant when compared to the tank heater. Liming is a serious problem in the indirect and difficult to fix. But be my guest. Regards TB