I've seen both versions and here are the differences between the two:
The PG-13 version is the theatrical version, widescreen. It contains a bit of language and suggestive dialogue--enough to make it PG-13.
The PG version is an "edited" version that was cleaned up enough to make it pass for PG, which is what the original creator wanted. IT IS FULL FRAME. Every site you go to will say it's widescreen, but it's NOT. It is full frame--so be aware of that. Other than that, the extras, DTS and DD5.1 sound, etc, is exactly the same as the original.
Most of the edits made in the PG version are stuff you won't notice because they just cut a scene a few seconds early. (For instance, in the PG-13 opening video, Melody is holding a sign that says "Honk if you love Pussycats," but the "cats" is cut off--in the PG version, you don't see this clip in the video at all. They replaced it with something else.) However, there are TWO bits of dialogue where they did voice-overs. Both times it's the bad guys ranting and you can tell their lips move differently from the "cleaner" dialogue that's inserted. It's just like watching a cleaned-up movie on TV in those two small parts.
I should also add that the ONLY place I could find online that had the PG version is Amazon. For both the PG and PG-13 version, they show the same picture, BUT instead of the purple "Widescreen" bar at the top, the PG version ACTUALLY has a pink band at the top that says "This Film Edited for Family Viewing." On the back it says 1.33:1 Full Frame.
Personally, I own the PG version because I prefer it--and the language is totally out of place in this film. But it's a sacrifice since you lose the widescreen.