There are so many ways to heat water that the above question is a formidable one.We will attempt to make it simple and compare the different systems with pros and cons.Also we will use scale system of 1-10, 10 being best.
                                 Pro :
Electric water heaters:  Can be installed in many areas of a dwelling as they need no 
     Score 3                venting.
                                 Con :
                                By far the most expensive way to heat water,especially
                                with the rates of Ct. power Co's.(save Wallingford electric)
                                Slowest recovery rate,(this is noticed when showering 
                                simultaneously,subsequently,or trying to fill a tub or
                                whirlpool, electric does not keep up.
                                                               
                                 
                                  Pro:
Gas water heaters:       Good efficiency.Clean operation for fossil fuel.If sized
                                properly will deliver well.
      Score 7.5               Con:
                                Needs to be vented to chimney or with a through the wall
                                system or power vent system;these types significantly raise
                                initial cost.

                                  Pro:
Oil fired water heaters:  Good efficiency. The best recovery rate and most deli-
                                very of hot water.If sized properly will handle any residential
      score 7.5              tub or whirlpool.
                                  Con:
                                 Much more expensive than comparable gas unit. An oil 
                                 burner (besides the main heating burner) that needs 
                                 maintenance. As with all oil equipment it needs thorough  
                                 cleaning yearly.And must be vented to chimney.
                                  
                                    Pro:
Aqua booster:              Very efficient as it uses existing heating equipment on boiler.
                                 Excellent recovery rate, very clean no venting required.One 
Score 9                       of the best ways to heat water.
                                    Con:
                                 Needs bronze circulator that is the heart of the operation.
                                 Easy to maintain but costs more than iron circulator.
                                      
                                    Pro:
In-Direct heater:           As with the above system this system works off the existing
                                 heating system.It has all the same benefits and may even 
Score 9                       surpass the recovery rate. A top way to heat water.
                                    Con:
                                 Initial start-up cost is higher than Aqua booster or gas 
                                 heaters.
                                    
                                    Pro:
On demand gas
water heaters:              No storage tank needed resulting in efficient delivery. 

score 4                                   Con:
                                 Very expensive start up cost.Difficult venting requirements, 
                                 troubleshooting can be laborious, does not yield enough
                                 gallons for average family with tubs or whirlpools requiring
                                 tandem systems which really raises start-up cost.(reports  
                                 have come in that it takes 45 minutes to fill a whirlpool
                                 with a single unit; I would forget that I was taking a bath 
                                 after 30!) Dangerous installations have been reported; un-
                                 licensed or untrained individuals.Stick with traditional 
                                 systems. Not worth the potential 10 or 20 percent savings
                                 on traditional gas heaters.

The above is based on decades of experience by myself. I am no scientist but I have installed and maintained these systems. By the way I use an Aqua booster at my home, in twelve years I have never run out of hot water!
           Thank's for the visit, Joseph James.