Well I wanted a low gear because I hurt my knee (chondromalacia-patella) while going up 7% climbs all day in a 25 inch gear (I still have the stock 50/39/30 tiagra crankset).
I guess there is little difference between 15.9 and 17.5 gear inches. I have a hollowtech compatible bottom bracket now, so I think a could get a hollowtech mtn crank.