It's actually a pretty tough gig... but if you were going to do it, you should do it for both... with a preference for Lyft.
Lyft are much kinder to their contractors, while Uber has a practice of some super shady business dealings, plus if you get into trouble Uber will not have your back. They're really just out to make a lot of money very quickly, and have financial backing from Saudi Arabia, which also, does not have a stellar record for treating workers right.
Both companies require you to work long hours, to make a normal wage, and bathroom breaks are hard. It's hard to find locations that you can take a bathroom break without purchasing anything.
If you get a long fare, you do make good money, but you also have to drive all the way back without making any money, so even that has it's downside. There will be wear and tear on your vehicle... I'm sure lots of Uber/Lyft drivers have decreased the value of their automobile significantly, and there is always the potential of danger, both from your passengers and getting into car accidents.
On the plus side, you do have completely flexible work hours and if you happen to live close to an airport then you could do really well for yourself.