The system may just need to be charged. The other possibility would be that it was way undersized, but the temperatures in your area are mild shouldn't be low enough where the performance of the HP really drops off. He should have an HVAC guy come out and take a look at it. If it is low on Freon, it also won't cool properly when summer comes around. During summer, you would see the indoor coil ice up instead of the outdoor one.
For cost, EH will always cost more. If it was very cold (say below 0), then you may want to just run on EH to save the compressor some work. Some newer models still produce heat (from the compressor) about half the cost of resistance heat at these low temperatures. However, the capacity also drops dramatically, so a bigger fraction of the heat comes from the resistance heaters as the outside temperature falls. In central FL, you should not have to worry about this.