We use ours for heat a little in spring/fall too. I know that in 50-60* air, they are probably cheap to run. I just hate hearing it run, so if it gets chilly at night, even in early June, I'll put a fire back in the boiler. I'd rather pay the coal guy than the elec company. Therefore, I am more concerned with cooling efficiency.
York's are somewhat pricey, fwiw, but I have had very good service from my 20 year old 3 ton 10 SEER York. [knocking on wood here] I have done absolutely nothing to it. I should have my butt kicked, but I have not as much as oiled the fan motor [should, every 5 years], nor have I ever cleaned the coil. [should, easy to do with the spray can cleaners]. I've done some reading, and York has two 14.5 SEER units that look like a good buy to me, as far as cost vs efficiency. I doubt that I would go higher than that.
Reading/studying, I have found that some experts recommend a single speed air handler [manually changed multi speed is ok], as the variable speed motors are very expensive to replace. Also, a scroll type compressor is supposed to be a good thing.
I'm still learning.