I don't think it is an ether/or solution. In some countries TomTom is better. I think that you could work out which is which and work out who to default to, or source both, regularly identify inconsistencies and have someone fix them before pushing out to the clients.
Ideally, yes, having two or three different sources would be great. The more, the better! Theoretically, Apple are already doing this, yet they seem to give a strong preference to TomTom data and ignore the rest. Officially TomTom update their maps four times a year, but in my illustrated case they failed to spot a significant road layout change that happened over a year ago.