Which amp will cause a larger increase in electric bill?

Actually, if an amp is pure Class A, it won't matter if it is push pull or not, assuming the power output is the same between the two.  

The reason is the maximum load current flows 100% of the time, so if the power output is the same, then the dissipated power at idle is the same.   What push pull does buy the designer, is it puts the dissipation across two complimentary devices stacked in tandem, which makes heatsink design easier.  However, the end buyer still pays the same electric bill. 

A number of amplifier companies define their products as Class A, when in reality they are Class A at lower power levels and Class AB at higher power levels.   I do note now that some amp companies state where their amps are Class A and where they become Class AB.   I like honesty in companies.

So, if you understand everything I just wrote, then realize that the only design difference between Class A and Class AB is the bias current and the impact that has on the heatsink and power supply.  

BTW, just to clarify, when I state Class A, I mean the entire amplifier is Class A.  There are quite a number of amplifiers that have Class AB output stages with Class A driver and amplification stages.   Those amps are still Class AB by IEEE definition.  

Thanks for responses.  So let me clarify.  If sitting idle (power on), which will cost more, a class A 100 watt (Threshold) vs a 250 watt A/AB (Threshold)?  I have both and a few more.  I'm not worried about electric bill, just wondering.

spatialking, thanks!

Thank you, gs 5556