Do new developers choose between programming in Windows 7 or Mac OS X 10.7? What are the benefits and limitations of each alternative?

It depends on personal preferences. Windows is good if you want to develop with .net but you can use C# on mono on Unix. OSX and Linux are much better for Open Source technologies and they have the typical Unix tools and commands (more than 1000). OSX has a great terminal, Bash or Zsh, iTerm is better terminal, Tmux is a great terminal multiplexer, vim and emacs are the most versatile, light and stable programmer's editors. So I find Windows quite limiting in general, especially for programming. I prefer Open Source software because you can read the code and learn from it if you want; you do not have to pay any license, you install it and use it; the community is open and collaborative and more of the innovation is happening there.

Neither. New developers are by definition untrained and start with the simplest type of programs possible, which is likely to be console-based (e.g. "hello world") such that platform differences between OS X and Windows does not matter. Moreover GUI programming brings too much complexities for people who just begun programming. As they move forward and starting their career, their first job would probably be along the lines of working in web applications and hence would likely dabble in JavaScript, PHP, or Ruby on Rails – due to the large demand for these jobs. Again the choice of OS X vs Windows won't matter much for programmers of these languages – it's primarily a matter of preference and other considerations. Some others would probably start their programming careers in office automation – so that will likely be Microsoft Office VBA macros and maybe to some extent AppleScript. If they're lucky they can make a career out of this and rise up the ranks. I know people who made their career this way by programming spreadsheets for traders in an investment bank and some of them just got their AVP promotions a few months ago – essentially became development managers for VBA programmers writing calculators for traders. These companies will likely use Windows, except probably some publishing houses that already have workflows built around AppleScript. So in conclusion, the choice of platform for new programmers does not matter since initially they start writing the simplest type of programs that has been around since computers have keyboards attached to them. Furthermore the paths of career choices from there usually starts at either the web or office automation. Should they go for the latter, Windows will be the more likely answer – but in the former the platform choice won't matter.