I've been using Lyft quite a bit this year, and I've asked every driver if they drive for both (yes) and which they prefer (Lyft, because it pays better). Anecdata, I know.
I have driven for both in the past. Lyft has major advantages in my mind:
1. Tipping is allowed so you make more money that way. I don't think the fares are significantly different (maybe they are, I haven't compared).
2. You can cash out early. I was driving because I had just gotten a divorce and moved out and had extra expenses with getting into a new place, and frequently, being able to put $50-100 into my bank account after a night's work was the difference between solvency and insolvency.
Also, I've observed anecdotally that the Lyft passengers are nicer. I've talked with various people about why that is and there are various theories, but generally the idea is that it starts at the top. Uber has a history of dick moves and I think that reflects in the different clientele.
They may be set up for both but if you probe an inch deeper I suspect you'll find they prefer and drive for one service much more. Which is why it's misleading to suggest that "they all drive for both".
I was in Chicago all last week and I asked this question of my drivers as well. Every driver prefers to drive Lyft because they make more money per ride, but they end up driving Uber most of the time because there's virtually no downtime. They can pick up the next person within close proximity and almost as soon as they've dropped off the last person.
Also, I've observed anecdotally that the Lyft passengers are nicer. I've talked with various people about why that is and there are various theories, but generally the idea is that it starts at the top. Uber has a history of dick moves and I think that reflects in the different clientele.