In general, both are incredibly reliable and resilient networks. Don’t conflate Openserve with Telkom of the yesteryear, these are two very different companies…
I’d say it depends on the cause of these outages. Looking at breaks: If your fibre runs along a route that’s prone to being dug up (one example that comes to mind is Olivedale, where a tiny servitude caters for 3 separate services with both Vuma’s pop and a sub station sharing an identical servitude for some reason)- then outages will likely happen every time a new power/fibre cable needs to be installed. If the repeated breaks are a result of a temporary construction (eg, widening of a road and workers digging up fibres), then you’re pretty much sorted once that’s done.
Both Vumatel and Openserve (and others) also operate their pops with sufficient backup power (in general) as well as protected backhauls (mostly). This means PoP outages are less frequent than they used to be, and both operate great backhaul networks with sufficient capacity.
Fundamentally, a tree can fall on your aerial fibre from Openserve or a spade can hit your trenched fibre from Vumatel. We say this to FTTB clients and I’ll say the same to home clients, especially those who work from home, no SLA beats having a good backup plan; so if your budget allows, get both.