I have always been unsure why people prefer one over the other - it's essentially the exact same service. I typically go with whatever one is closer / is offering a deal at the time.
I think they have distinct experiences. Lyft drivers are more talkative and prefer you sit in the front, with Uber the status quo is get in the back and conversation is optional.
I check both apps, if the pick up time and price is similar I'll take Uber.