Rendering directly to QuickTime is fine for short, quickly rendering, disposable test pieces. Rendering to TGA is recommended for final render as it's a non-lossy format - which means that when you do create a compressed movie from those TGAs, you're starting from pristine images. Another reason for rendering to TGA is that should the rendering process be interrupted (power cut, cat throwing up on the keyboard, etc) then at worst you've lost one frame, and can restart from where you left off - if you're rendering to a QT file you lose the lot.
As for creating a movie file from those TGAs... I coughed up for a QT Pro key, which lets you import a sequence of images and export using any of a wide selection of codecs. You could, of course, import your TGA image sequence back into A:M, apply it as an animated decal to a flat plane, point a camera at it, and render that out as a QT file.