I would stay away for the 56/42... Most people can't really use such high gear (espcially if the rear cogs are 11-23's or even 12-25's). If you have some hills (and it sounds like you do), I would stick with a 53/39.. Tried a 56/42, but my cadence suffered to the point where I lost time, instead of gained time... You can always change the rings though if you find you're not happy with the combo.. so not a big deal to try it....

Swedish triathlete Björn Andersson rides a 58/11 as top gear, but cadence is only around 80... really crunching the pedals...