That depends on the cogs you choose. As you increase the number of cogs, you gain some ability to accomplish both things at once. Ride your bike a few times, taking note of which gears you use most often. Also notice if there are gaps in your current setup where you think "gee, wouldn't it be nice if there was something in between these two?" Then play around with this (courtesy of Sheldon Brown, once again) to see what would be ideal, then shop for something that comes close. Also remember that it's best to avoid the extremes of inner chainring-outer cog and outer chainring-inner cog.