I guess it depends on how you feel on those hillier climbs and how often you do them. Do you feel like you could use a lot of "help" when climbing on your current bike? If so, then maybe you want to go for the 12-29. If not, then the 12-27 is likely fine.
Also, is your new bike significantly lighter than your old bike? If so, that also might make the climbing a little easier too.