There are pros and cons to both and I prefer to run a double stepped hub so I can run fixed / fixed as it is always nice to have a few gear choices.

Descents on a fixed gear (at speed require some pretty mad skills but with practice, and a tall enough gear you can descend at close to 40 mph... this requires an 80 plus gearing and the ability to spin in excess of 160 rpm.

This will also limit to get up the other side unless you are a freak.

For commuting I'd look for a gearing in the low 70's for the fixed side and use the ss as a bailout and descending gear with a gearing in the mid 60's... running a tall gear into the wind for 10 or 20 miles sucks but something in the low 70's will let you attack moderate hills with relative ease.

I have tried to find some love for a fixed / ss combo but with the exception of my path bike prefer that my bikes that coast are also multi geared.