Since the performance is the same regardless of what scripting language you use (This has been officially confirmed AFAIK) I usually just use Unityscript (Javascript) since most Unity examples are in javascript.
Just add #pragma strict at the top of your .js scripts to switch off dynamic typing. Voila, easy to debug unityscript. I'm learning C# coming from an as3 background though, since it seems like a more generally useful language overall. Only so much room in my poor artist brain for computer languages.
@Talon: C# isn't all that different imho. Especially if you have a background in C or Javascript. I always thought of it as a type-safe version of Javascript. Granted, some of the LINQ and Lambda syntax can be a bit daunting, but you get used to it. Speed-wise, uscript scripts compile to a mono/.NET assembly, and so do the…