Single speed is fine for commuting -- I did it for a couple of years. Well, fixed and single.
Ratio I like is closer to 60" than 70" -- mostly because I don't mind coasting (on SS obviously), and when it's windy a 70+ gear sucks in my opinion. Actually, so does a 60 - but it's more tolerable.
I use gears now. The novelty of SS wore off, and I like to carry loads on my bikes
.