They both matter, but cost is king, and is a function of lots of things including efficiency. The cost for obtaining 1 kWh from a 90% efficient panel that last

They both matter, but cost is king, and is a function of lots of things including efficiency. The cost for obtaining 1 kWh from a 90% efficient panel that lasts 1 year will be much greater than the /kWh cost of a 40% panel that lasts 10 years.