As alluded to in the write-up, Light Years is a much warmer, more human album. It's unashamedly camp and a total celebration of disco culture. Fever is the exact reverse of the coin - cool, sophisticated, clubby, borderline emotionless, often robotic in its adherence to rhythm. It feels more like a concept album in some ways. Both contain her greatest singles, demonstrate pretty astonishing quality control and have aged really well.
I'm not sure I can choose a favourite really