I like steel: Mark V.
As for gearing, I presently ride a 53X19, for 77 gear inches.
I have about 800' of dynamic range on my 13.5 mile commute, which includes a short killer hill and one long grind of about 1.5 miles.
I also ride with a front brake, which makes a difference if it gets away from me going downhill.
Put the above in the context of riding fixed gear 30 miles per day for the past 1.5-2 years.
The above said, I find that 72 gear inches gives me the best all-around performance, especially when one considers controlling the bike downhill and braking with the legs only, and without any tricks such as skidding.
If I could send myself a message with a time machine, I would advise myself to start out with 72 inches, and maybe even stay there.
How does one figure gear inches?
Select the proper wheel size; in my case, 700 X 23.
In the gear units
field, select Gear Inches.
In the chainrings
field, I put 47, 48 and 53.
In the custom cassette
(cog) field, I put 14, 15, 16, 17, 18, 19.
Then, hit calculate
47 X 17 = 72.7 gear inches.
On my 13.5 mile commute, I find that I get identical times (41 minutes) in either 72.7 gear inches or 77 gear inches.
I like the sensation of extra speed I get with 77 inches, and I like the sense of extra downhill control I get with 72.7 inches.
They both produce the same time on my commute.
For further comparison, a lot of factory fixies come equipped with 48 X 16 chainring and cog, for 78.8 gear inches.