Honda's design - Integrated Motor Assist system uses an electric motor between the engine and continuously variable automatic transmission (CVT) to aid propulsion and turn the 1.3-liter i-VTEC 4-cylinder engine off at stoplights. Integrated Motor Assist is a "mild," or "power assist," hybrid system because the electric motor can't propel the car on its own from a stop, though it can do so for a short time in some steady cruising situations.
Honda's EcoAssist, has guidance and scoring functions, as well as an "ECO" mode that dulls throttle response to improve mileage. The guidance function includes a digital speedometer that turns green when you are driving efficiently and blue when you are hard on the gas.
The gasoline engine delivers maximum power of 69Kw at 6000 rpm. It uses 2 spark plugs per cylinder. The two spark plugs in each cylinder can fire either sequentially or simultaneously, enabling more efficient burning during lean-burn mode and more often lean-burn operation. The VTEC cylinder idling system of the engine closes the valves in three of the four cylinders when the car is decelerating, reduces the power lost to the engine by 50 percent and allows the IMA to extract more electrical energy during braking. The rocker arms operating the intake and exhaust valves have two modes: valve-lift mode or idle mode. They are engaged via a synchronizing piston. During deceleration, the synchro piston disengages the lift-mode rocker arm so that the valves remain at rest, effectively sealing off the cylinder.
Toyota's design - Hybrid Synergy Drive system uses a 73-Kw 1.8-liter (ZVW30) 4-cylinder engine aided by two electric motors, one of which assists the engine when more power is needed. Toyota quotes total output at 100 KW and acceleration of zero to 100 kph in 9.8 seconds.
Hybrid Synergy Drive is a full hybrid system because the motor can propel the vehicle up to 42 kph without the aid of the engine. The Prius also has an "EV" button that allows the car to be driven on electric power alone. The upshot is that drivers can aim to keep the engine off as often as possible.
The Atkinson cycle engine delivers maximum 73Kw power at 5000 rpm and only starts once the vehicle has passed a certain speed and after starting operates in a narrow speed band.
Conclusion : The electric motor will still be operating at peak capacity long after the engine has given up so maintaining efficiency will depend on the durability of the petrol engine.
Without a doubt the engine in the Honda works much harder. In the Prius the engine only operates from 2500 rpm - 5000 rpm, wheres in the Honda it is working from 0 rpm to 6000 rpm. This means that a Honda engine that has done 100,000 Kms would have significantly more wear and tear than the Prius which would probably only show the wear and tear of a vehicle that has done about 50,000 Kms.
The Honda engine is more complicated and uses 8 spark plugs. This just means more things to go wrong and higher maintenance cost.
We must bear in mind that all comparisons are done with vehicle in peak condition and this is only true when the vehicle is new. My opinion is that overall Toyota's design is far superior. It will still be running perfectly (like new) even after the vehicle has done 500,000 Kms, and it will cost a lot less to maintain.