Originally Posted by
caloso
39 x 27 = 38.0 gear inches
38 x 25 = 39.9 gear inches
Since you'll eventually wear out and need to replace your cassette (and you'll rarely need to replace a chainwheel), it makes more sense to get a new cassette with a bigger cog than to get a newer, smaller chainwheel.
Oh I just meant get the 12-27 now and if I still need lower gearing for hills add a 38 on eventually as well.