Hmmm, well .. the last time I used pro tools was about 5 years ago, so I'm not sure what the new version is like ... though I'm pretty sure you won't find STP to be a suitable replacement.
Frankly, STP is not up to pro standards, at least IMHO ... it has a lot of great features that are highly usable, but I don't think it's up to the task of creating complex scores ... it's too heavy and clunky to work with (try putting 20 or 30 tracks with effects on them through STP), and there's also no MIDI, which will limit you severally. I personally only use it for simple scores or occasionally to clean up some audio.
On the flip side, STP is an AWESOME tool for analyzing and repairing audio ... but really, that's the only area that it excels.
Personally, I'd look at Logic ... While the video side of Logic is not as well developed as it could (should) be, it's usable for most situations, and you can't beat Logic for the pure act of making music and soundtracks ... Logic is an incredible audio tool.