Yeah, both is the copout answer here

Obviously you would like to win both. But if you could only have one...
For personal vindictive reasons the Big 12.
For the long haul growth of the program it has to be Big 10.
10 years from now who is really going to care that we won the Championship game of a conference that probably isn’t going to exist. On the flipside as a member of the Big 10 we can always say that we won the inaugural Big 10 Championship game forever. It would also be very pleasing to go straight into that league and jack them upside the head and show them that there is a new sheriff in town.
So Big 10 title gets my vote.
The only argument for Big 10 IMO is if a win in the inaugural championship is necessary to springboard long-term success (as it did for Texas). I don't think it is. I think we are climbing towards long term success anyways and aren't in the kind of position that needs a springboard. We are already out there in the spotlight, and the program is already in the process of being built.
In the long run, bragging rights and historical prestige are going to be what matter most, and so our legacy will depend a lot on Big 10 performance in the long run, not just the first year.
On the other hand, Nebraska's time in the Big 12 could always be viewed as one where we won just 2 title games in its existence, or one where we were a dominant power that dropped off for a while but finished off the conference with a bang. Intangibly a win to exit the Big 12 - or perhaps more significantly, avoiding an unceremonious loss to send us on our way out the door - is more important than how we do in Year 1 in the Big 10. If we don't make a splash in Year 1 or lose the title game or not reach it, there's always Year 2 and 3 to establish ourselves immediately there, but if we actually lose this last Big 12 title game, that's an indelible mark. OU will have kicked us out of the conference. So I really hope we win. We got a lot more riding on this game than we do on next year's hypothetical one, even if it becomes reality.