As far as customer service goes, they are all bad.
MTN has historically been pretty good with broadband and coverage, especially when it comes to speed, but their service has degraded severely over the last few years. I'd say Vodacom wins on signal, for the most part.
Your specific location may be different, but MTN in our area is notoriously unreliable. I've reported issues several times, and it's always apparently my devices at fault, even though they work perfectly fine on Vodacom. Rain has decent signal here as well, but not MTN.
If you go 2-3 km from where we live, you will struggle to open a web page on your phone with either Vodacom or MTN at some times of the day, but usually, you can make calls on the Vodacom in the same area, but not on MTN - so still a bit worse.
Oh, and I live in an urban area. Not in the middle of nowhere.
Get a prepaid SIM from each provider and test.