I don't "think" in gain ratio
, and prefer gear inches
However, I will go to Sheldon Browns Gear Calculator and make a comparison:
I ride with a 53t chainring and with various cogs: 17t, 18t, and 19t.
53 X 17 = 81.9 gear inches
, or a 6.1 gain ratio
53 X 18 = 77.4 gear inches and a 5.8 gain ratio
53 X 19 = 73.3 gear inches and a 5.5 gain ratio
53 X 20 = 69.6 gear inches and a 5.2 gain ratio
From my experience, in my riding environment, 73.3/5.5 gives the best all around uphill and downhill performance.
I really like the feeling of downhill control I have at 73.3/5.5.
However, I commute about 30 miles round trip per day, and I consider the extra speed I get at 77.4/5.8 a fair gain for the downhill control I loose.
I have ridden comfortably at 81.9/6.1, but I have almost no downhill control.
I went back to 77.4/5.8 NOT
for the uphill ease but for the downhill control.
If I lived in an urban area with denser traffic and more hills, I would go with 69.6/5.2.
Regardless of ratio I have no difficulities going uphill.
The higher ratios affect the downhill portion much more than the uphill.
If I had to live with just one ratio for the rest of my life, I would probably go with 73.3/5.5 for the all around performance, and especially the downhill control.
Follow the link above.
Make sure to enter your correct tire and crank size.
Enter the chainring you expect to use (I have four different chain rings) and the various cogs above and below what you think you might use (I also have four different cogs, all EAI).
If you want to go fast downhill with a 5.5, pull up on the pedals with your feet and put more weight on the saddle, and your spin will increase dramatically with less or no "bobbing."
For myself, I find it more fun to go downhill in control than to go fast.
I save fast for uphill.