I would use the Shimano for a few reasons - the length isn't going to make much difference, Shimano chainrings shift much better than FSA chainrings IME, and they are a wee bit lighter (although I doubt this will make the least bit of difference).
You body needs to output power to keep your bike moving through the wind or up a hill, and the amount of power it takes to ride up a is the same if your legs are making a circle with a 165mm radius or a 170mm radius, and the power you can produce is pretty constant if you take the time to get used to minor differences.