i wouldnt use a fixed cog without a lockring.

i probably wont ever use a flip-flop hub ever again since the only difference between fixed-fixed and fixed-free is that you have LESS options with FIXED-FREE. unless a quarter inch of threads is aesthetically unappealing...go with fixed-fixed.

on my other bike i have the free cog 2 teeth larger (about 10 gearinches less) and i love it. your intuition is right...less gear inches on the free side lets you spin when you want and coast when it gets to be too much.