46/18 is a really good gear to spin with. Id say that 42/18 is going to be to low. You will not like it on the flats or downhill at all. Id suggest stick with the 46/18 for now.

I ride 52/16. It makes the climbs a little torture some but its a really nice gear otherwise. I did experience knee pain but found out it was just an adjustment in my cleats.