If I get a chance, I'll try to post some figures relating the speed that would result from the gear ranges at certain cadences.

For most people the bigger concern would be having a low enough gear to climb in the Sierra foothills = -18% gearing. Then a secondary question would be how fast you want to go on the flats. 20 MPH? 25 MPH? It is pretty easy to calculate the speed resulting from the top end gear at your preferred cadence.

My personal choice in your situation would be the 6-speed - 12% gearing, which is what I have on my bike, used primarily for travel. The lowest gear on my bike gets used quite a bit, the highest gear, not very often. I'm not a racer but am a fairly fit rider, and tend to glide on downhills rather than push for speed. I am rarely interested in going faster than about 18 MPH on the flats, but can go much faster if I choose to.