I would try the 12-27 swap before the 11-34 (and even ask the LBS to make this change free of charge at the time of purchase). I use both of these casettes, but prefer the 12-27 since the gaps between gears starts to get too big on the 11-34 which can degrade performance a lot. I expect you wil find this bike climbs so much better since its lighter that the 12-27 will be fine. If it isn't I would swap my smallest ring in front for something smaller before going to the 11-34 in back (you could also do this at point of purchase to for more bargaining power). Note, that if you go the 11-34 (or 32) roote in back, you'll also need a longer cage rear dearailleur.