FYI: There are compact cranksets with carbon crankarms. I have one, FSA's SL-K compact. They are available. Did the shop just not want to order one for you?

In terms of gear inches, there isn't much difference between a 34 chainring and a 27 cog (road setup) and a 39 chainring and 32 cog (road/mtn setup): 33 vs. 32 gear inches. One big difference between the two set-ups is the weight, however, which may more than negate the slight gearing advantage the road/mtn setup has.