Lior,
Note this is a UK based forum, so some model numbers are different here, as well as prices...
But you've got 3 good models on your short list. The Canon Optura 600 is the Mvx4i here, and that is discontinued... not sure about in the US. You might also want to add the Canon HV10 to your list - it is their new HDV camcorder, similar in price and cost to the Sony HC3; similar in looks/feel to the Optura 600/MVX4i.
A good place to look for answers to your questions is here (which is US based), all 4 of these cams are in the top 8:
http://www.camcorderinfo.com/ratings.php
The HD models (HC3 and HV10) have the edge in picture quality, especially if you have (or will ever get) a HD TV/display. They are very close in general video quality; the HC3 has a big edge in low light. For stills, the HC3 is the clear winner. Audio: The HC3 can take an external mic (though only one which supports the Sony AIS shoe), the HV10 can't take an external mic at all. The GS500 has a standard mic jack so wins here. Editing: HDV (HC3 and HV10) is "harder" (in terms of PC requirements) to edit than DV, though with a resonsbly fast PC (Pentium 4 3.0 Ghz) you can edit it directly. You can edit on a slower PC using a "proxy" or "intermediate" (i.e. it can be done but requires an extra step). But I think the extra requirements or hassle of editing HD is worth it.
Between the HC3 and HV10 it is a close race... but as it has the edge over the HV10 in:
Video at dark places quality
Still quality
Sound quality
I'd recommend the Sony HC3.