I learned Java in about a week from C++
So all these silly Java developers should just lump it and learn Objective C.
The advantage is, depending upon how it's done in the background, it might be a lot easier to write portable code using C# and Mono than in Java (and having to write device specific GUI code etc.).
Look, there are advantages to both languages, but personally I find that C# makes for far more straightforward code, whereas Java seems to lend itself to sprawling projects with obscure classes. At the end of the day, as a Java developer there are things that you are used to that you'll argue to the death about (such as operator overloading) and there are things that I'll argue about because I have got used to them (such as operator overloading), I don't think either viewpoint is right, but I think the cross-platform possibilities of a Mono implementation are interesting at least.
As to VS vs Eclipse, it's a matter of taste again. Personally I find the comparison to be very close, but I prefer VS, purely because I work in it day to day.