Notwithstanding the above comments, a hybrid (diesel/electric) system generally involves more 'inefficiencies' (more components each with their own efficiencies) though regenerative braking (where the energy is returned to battery power rather than dissipating it as heat by the brakes) can improve the overall efficiency.

Logically (!) city use with stop/start operation probably better-suits hybrids than sustained high-speed long-distance runs.

Of course the efficiency of the generating component (the power source) is significant. A gas-turbine (as used on the DesignLine vehicles) can be efficient when run at constant speed (as can a suitably specified diesel engine run at optimum (fuel economy) speed).
Attempts in the 1960s (by Ford and Leyland - and Austin in the 1950s) to power road vehicles directly by gas-turbine engines ended in failure due to the fuel consumption of variable-speed operation.
It is the variation in power demand (and, therefore, revolutions) that causes variation in fuel consumption.
A poorly-specified power-source (running constantly at peak power) might be inefficient and 'hungry' for fuel and worse than a simple diesel/mechanical transmission.
Topping-up the traction battery from mains electric will reduce liquid (or gas) consumption and cost will depend on the tariff of the mains electricity used (and might involve additional manpower (and time) to achieve 'refuelling').
Consider the complication of installing a suitable charging point and the time required to charge a simple domestic car that runs solely off electric battery power versus driving to a pump and refuelling liquid (or gas) fuel.
I understand that some gas-powered buses in the region have to be driven significant dead-mileages to access a refuelling point.

The following was gleaned from an overseas source last year (and might not represent current technologies and prices):-

The resulting economic analysis suggested that procurement of a hybrid bus in lieu of a B12 will deliver a negative economic outcome, with a projected loss of $113,950 (net present value basis) over the life of the vehicle, and a benefit-cost ratio of 0.84.
Compared with a bus fitted with contemporary B7 technology, the economic outcomes are more negative than for the B12, with a projected loss of $313,920 over the life of the vehicle (net present value basis), and a benefit-cost ratio of 0.10.
A sensitivity analysis revealed that the quantum of the above losses would be partly reduced in the face of substantial rises in diesel fuel prices, but the overall economic outcomes would likely remain negative under even more aggressive fuel price rise scenarios.
While the hybrid technology (as trialled) delivered a fuel efficiency benefit of around 15% relative to conventional B12 technologies, this benefit was not sufficient to offset the higher capital cost and ongoing maintenance costs of the hybrid bus. It is estimated that an 88% reduction in the capital cost of the hybrid bus technology would be required to support a break-even investment when compared with a traditional diesel bus operating 70,000 km p.a. over 25 years.
In the case of the more contemporary B7 bus technologies, the hybrid technology (as trialled) was less fuel efficient. This factor, coupled with the higher capital and maintenance costs over a typical bus life, suggests that the B7 currently delivers superior operational, environmental and economic outcomes when compared with hybrid bus technologies.