Originally Posted by
spelger
essentially...big chain ring/small cassette gear is easier going down hill, small chain ring/big cassette gear easier going up hill.
-scott
This isn't entirely accurate. Lower gear ratio (chainring:cog) will always mean easier pedalling, no matter up hill or down hill. It's just that you are far more likely to spin out going down hill.
There is one other nuance: if the climb is short and you speed up before it, the momentum will make more difference than the gear you are in - i.e., the effort you will put in using, say, 48/16 or 38/16 will be very similar, but the speed in the latter case will be lower.