Originally Posted by American Euchre

Let me rephrase my concern: you are in a lower cog when in a 50 compared to 42 or 39 (at the same speed), therefore, more likely to run out of gears when downshifting in coming to a stop so you can start up again comfortably.

This means you are more likely to be either cross chained or forced to downshift to the smaller ring. Workable but less than ideal. Neither is likely to happen with a 42 or 39.