this has been discussed before (not recently) and i'll sum it for for ya
#1: Maintenance - single speed has less **** to maintain and potentially break on ya. point 1 for SS
#2: Terrain - if your SS is geared for flats, much more difficult to mash up large hills, much less mountains (colorado); if it's geared for hills/mountains, you just spin spin spin on the flats and go nowhere. geared bike has more gears, allowing you to tackle more varied terrain more efficiently. point 1 for gears
#3: Shifting - SS doesn't. on geared bike, gotta think "do i shift now? down one? down two?" instead of "just pedal" doesn't seem to to be that big of a deal, but point 2 for SS
#4: Speed - geared bike, as mentioned above, with identical rider is always going to be faster, with the exception of a velodrome. point 2 for gears.
so it really just matters which of these 4 things are more important for you.