Should I get a smaller chainring or bigger cog? I built a single speed bike about a year ago, and am starting to change out parts to make it more comfortable for me. The first thing I'm wanting to change is the gear inches, which I didn't give much thought to at the time of riding. I have a 700x25c wheel with a 16t cog , and a 46T chainring that also has a 165mm crank arm. If I calculated the gear inches right, it's 79.2 in. I'm in a pretty hilly area, and I'm wanting to see what effect having a gear inch more around 70 in will have. I'm thinking of either getting a chainring that is 40 teeth or getting a cog that is 18 teeth. Is there any argument for which would be better to replace? Also, is there anything else I should be thinking about or considering when purchasing them?
For the same gear ratio, more teeth each end means:


More weight
Weaker chainring
More chain
Reduced angle at each chain pin as it wraps round the sprocket.
Potentially reduced vibration (this is complicated, I'll work on an edit later)