Don't destroy Bitcoin or block the development.

Core: Allow a limited increase of the blocksize. Lightning is far away from operational. Until then let Bitcoin naturally grow and be the number 1.

BU: Accept a limited increase of the blocksize and accept SegWit. Give core time to show that their concepts work and make sense. Until that do not fork the chain.

Core and BU: make a 12 or 24 months pactum de non petendo. After that period you can start again to negotiate.