I've never waited too long for a bus, but at Epcot and Animal Kingdom I just like to be able to jump in a car immediately without waiting, then dealing with everyone getting on the bus, and having a little more personal space.lol We did use the bus a couple of times too, just really depended on our mood and exhaustion level.
With Uber they do not take cash for the ride itself, you order on the app and it automatically charges your card after you complete your ride. It's nice because you know the cost up front, there is not a meter....but be aware -the price can go up in a matter of minutes apparently on the app. This happened to us when we used Uber from our resort to the airport, I was kind of shocked bc it literally was within 2 minutes of the first time I checked. This is only before you have selected a car. The price will not go up once you have reserved a car though, only when you are looking. (The airport ride was the one time that I noticed this, so for travel within WDW you shouldn't have that same experience)
How it works- you open the app and put in where you want to go, it will show you all of the cars in your area and approximate arrival times. Then it shows you the name and type of car of the person closest to you, then you choose to reserve or not. You then get a call from Uber with the driver on the line, when at the parks typically they will tell you they are already outside of the gates and give a more detailed description of their car.
I think right now there is a "first ride free" special. I know there is always some kind of promotion, so your first ride will be free.
You can not tip on the credit card though, the driver's can accept cash tips only.