The cog spacing on all 11sp groups are close enough, so you *could* go Campy using a Shimano or SRAM 11sp cassette. You didn't really give us enough info to make a recommendation: Are you buying a full groupset or just the relevant 11sp parts? Are you buying new or used?

Looking at new groupsets Ultegra Di2 6870 is the same price as DA 9000 ($1300) and I'd rather be on Di2. Chorus 11sp is $1200. As others have mentioned Ultegra 6800 is totally the best bang for the buck out there right now at $700. Maybe put the $600 price difference towards some sweet new wheels?

OR

Just get a Force 22 front derailleur and enjoy the benefits of YAW shifting without moving to 11sp.