11-27 or 11-28 Gear set

I need a little help choosing a cassette. I might have asked this at an earlier time but cant remember or find the post. Ive got a Cervelo P2 with 50/34 up front and 11-25 in the rear. Im doing IMLP and it was suggested to get 11-27 for climbing and the 28 will have to much spacing between gears  but when I called the bike shop they told me to get the 11-28. I want to be able to spin up hills a little easier but still have the gearing for fast flat racing. Any thoughts would be appreciated. Ive been working my A@# off this winter with Trainerroad and Sufferfest. I want my bike to be as strong as possible to set me up for a good run. This will be my first run of 26.2 miles so I figured I better build the bike for the run. Thanks.