Vim: Use "if" in augroup?

I just want to define an autocommand that executes some external commands. For example, delete some auto-generated files after compilation. I work on both Windows and Linux, and the external commands used are different on these two platforms.

Therefore, I can define:

    if has("win32")
        autocmd foo bar_win
    else
        autocmd foo bar_nix
    endif

If I want to define this autocommand within a augroup, should I simply put this piece of code into an augroup like:

    augroup fooo
        autocmd!
        if has("win32")
            autocmd foo bar_win
        else
            autocmd foo bar_nix
        endif
    augroup END

I tried this method, and it seems this works. I am wondering if this is the correct way to do it. Another way I can think about is to write the external commands into a function within which I can use if statement. Then define autocommand in the usual way and call this function. Which is better?

Thanks!