I have been using Taxify & Uber almost daily since about mid December.
TL: DR
Taxify is cheaper, but Uber is better.
On longer trips Taxify is on average R40 cheaper than Uber (without surge pricing). Taxify does not have a lot of surge pricing but is starting to experiment and will be introducing it.
For shorter trips I always use Uber for the reliability as the pricing is much the same as Taxify.
The same drivers normally work for both Taxify & Uber if the car qualifies. Uber is stricter on their requirements insisting on higher standards of both vehicle and drivers.
With Taxify you WILL be frustrated - Uber is reliable, Taxify is not.
You will very often have Taxify drivers accept your ride and then 5 minutes of waiting later will just cancel the trip, wasting your time. This happens very often. Also, they will accept the trip and just not move towards the pickup location - stalling to see if they perhaps get another trip closer to their location or perhaps an Uber trip at a higher rate (remember they mostly ride for both)
It also happens often with Taxify that you they will accept the trip & your wait time is 5 minutes indicated, but ends up being much longer - then the driver (after picking you up) will also stop at a petrol station to fill up first. Happens very often.
You can report issues on the Taxify app relating to your trip, but you will always get exactly the same response - word for word - irrespective of the problem you reported.
Lately on Taxify I have also experienced where the driver indicates he has arrived and is nowhere in sight. If you look at the map and car movements it seems like the driver is spoofing his location. I take photos of the cars when this happens to prove that the driver never arrived.
Once I also had a Taxify driver not end the trip and carry on driving while "on the clock". When I called him, he said he will end the ride and hung up on me.
Taxify: Unreliable, dodgy drivers & practices (legacy taxi driver tricks), older cars, bad after sales service. Cheaper on longer trips. Price estimates only.
Uber: Reliable, higher standard drivers & vehicles, higher prices consistently, crazy surge pricing. You know what you pay for upfront.
From a driver's perspective Uber is better - although they pay a much higher commission. I think Uber is 25% and Taxify is 15%. They however get much better support from Uber when things go wrong with rides, etc - like when riders don't pay for cash trips, etc.
Uber will assist and help them with problems, with Taxify they pretty much have to sort things out themselves.
Taxify is cheaper for the rider & driver, Uber is better for the rider & driver.
Before the ride starts, with the Taxify App the driver cannot see if it a cash trip or a longer trip, with Uber they can.