Alvin,
I'm a still photographer as well (mostly digital, Canon10D, and a bit of 35mm B&W), and I'm the same way about image quality.
I've shot extensively with the GL2 and DVX-100 (and 100A). I agree, knock the GL2 off the list. The XL2 is closer in line price-wise with the Sony and 100, and actually features 24p.
If you're thinking of using this for videography of nature subjects, the XL2 may be an excellent option for you, if you're already bought into the Canon line of glass. You can get an adapter for the XL2 (and XL1, previous model, no 24p) that allows you to mount any Canon EOS line of lenses on it, even up to the 1200mm. (Put that together with the mag factor of the smaller sensor and it's something like a 1700mm, I think).
The Sony doesn't shoot true 24p - it has a "fake" 24 frame progressive "look" option. That won't help you with the transfer to film - but the enhanced resolution will.
The XL2 and DVX-100 both feature true 24p progressive scan. They can handle blowing up to 16mm fairly well, and if things are well-lit, CAN be blown up to 35mm - just doesn't usually turn out amazingly well.
If you've seen the documentary Supersize Me, that was shot on the Sony PD-150 PAL (25 frames per second, non-progressive). A lot depends on how much you spend in the transfer process from DV to film.
Lastly, contrary to popular belief, shotguns aren't as directional as people think. Even for nature documentary a decent shotgun will have quite a large pick-up pattern. Unless you put a dish on it, of course.
Feel free to drop me a line if you need more help.