MacOS – Understanding the Difference Between ‘Activate’ and ‘Launch’

How about this: It's not exactly what you want, but achieves roughly the same thing.

I seem to remember pre-OS X you could activate an app and keep it from being frontmost, but that appears not to be possible these days (or I imagined it.)

to replaceFrontQTPlayerWithFile(aFile)

tell application "QuickTime Player"
    activate

    try
        set frontDoc to front document
        close front document
    on error err number errNum
        if errNum is -1719 then
            -- There is no open document
        else if errNum is -10000 then
            -- Front doc exists, but does not really...
        else
            log err
        end if
    end try

    open aFile
    play front document

    -- Hide QTP
    tell application "System Events"
        keystroke "h" using command down
    end tell

end tell
end replaceFrontQTPlayerWithFile


on run
    set unixFile to "/Users/diggory/Music/iTunes/iTunes Music/Music/Underworld/Oblivion With Bells/01 Crocodile.m4a"
    set macFile to POSIX file unixFile
    set fileRef to (macFile as alias)
    my replaceFrontQTPlayerWithFile(fileRef)
end run

Both shell- and AppleScripts can be used just about anywhere either is appropriate.

AppleScripts work better when talking to apps and user-level system facilities. (For instance, you can say tell app "iTunes" to play and it will, or tell app "Finder" to open the first file of the second window.)

Shell scripts (you referred to bash) work better when talking to low-level system objects and Unixy stuff.

They're both interoperable: you can call a shell command from AppleScript with do shell script "command here" and you can call AppleScript code from the shell with osascript -e "AppleScript". Further, there's quite a bit of overlap between the two (especially when it comes to file system operations).

There are certainly more resources for learning shell scripting online than AppleScript and it doesn't help that AppleScript has a reputation for being a "read-only" language (i.e., harder to write than read).

In any case, the most modern way to do scripting is with Automator, where you can use either shell- or AppleScripting (or both) along with pre-built modules from various apps you have installed.