The nice people over at the aforementioned Backblaze have some data on
SSD and
spinning rust drives. The jury is still out on which is more reliable for their use case, but the failure rates overlap. Their operating regime is different to a home user's.
It seems that, in practice, wear life isn't what kills a modern SSD.
Their numbers though (average drive life of under 3 years) means you'd be expecting something like £60/year for a two drive backup, and there is a choice of backup providers at that price point.