Try rendering into single images, then you can use "open image sequence " if you have Quicktime Pro. Rendering to an avi, compressing it there and, then recompressing it in Quicktime can create artefacts. By the way - this are not file formats, Quicktime and AVIs are only containers, you also can play an avi in Quicktime if the right compression codec is present. But there are some codecs which do not play crossplatform.