I've said it many times, I wouldn't rely on the mobile network for home broadband personally.
Its fine most of the time, but when a tower has problems it tends to go on for months.
My initial impression was Vodafone was really stable here, until it wasn't. Three was rock-solid for the first two months on my contract, then it wasn't. By the nature of how many customers are running off the same tower its just much harder to maintain consistent performance, and you have the bonus factor of weather can impact your signal.
The issues Zen have had in the time I've been with them have paled in comparison to that, as mostly they never impacted just basic web browsing whereas the mobile problems have.
If you can afford to do so I'd stick to what you have been doing, FTTC for main connection with mobile as a backup in case of problems.