since you've been making films for a long time, i'll categorize you in the advanced column.
but since you're a beginner on the mac, start out with the simplest option, iMovie. not as a tool to make a full length picture, but as something to smooth your transition into the mac and video. you can practice capturing, editing, and some of the basics.
once you're more comfortable, go for Final Cut Pro 2.
that's what i use. i previously used Premier 4.2.1 but version 5 gave me a headache, so i ditched it. final cut is powerful, easy to use right out of the box for basic editing needs, and you know that apple is going to keep upgrading it.
do you already have a machine? need advice on HD brands? speeds? etc..