"Monospaced" is the default, I believe. You should be able to just delete that line from the preferences and have it default to the default, so to speak. I think.
You really want a Fixed-Width font for coding (like Courier, and NOT like Times.) There have been some articles about this, for example: Dan Benjamin
I believe that there are also complaints about the way the IDE renders fonts; not doing smoothing, or somesuch. I tried some of the fonts mentioned in the article above on my mac, and they didn't look very good.