1) 72% depth will adversely affect the brilliance. Light will get caught inside the stone.
2) Good symmetry isn’t a problem. you can’t tell the difference between good, vg, or ex symmetry or polish.
3) Variance in girdle thickness is completely normal
4) Yes, it’s true about the halo masking a slight rectangular shape.
5) The XX probably adds to the price. Additionally, the other stone is probably relatively cheap for it’s color and clarity because of the big depth.

The truth is, though, that 13K for a G VS2 is expensive. See these better value alternatives:
http://www.jamesallen.com/diamonds/F-VS2-Ideal-Cut-Cushion-Diamond-1398012.asp
http://www.jamesallen.com/diamonds/F-VS2-Ideal-Cut-Cushion-Diamond-1353115.asp

Those are just for direct comparisons sake, but if you’d ask me, I’d recommend this stone for the same money:
http://www.jamesallen.com/diamonds/G-SI2-Ideal-Cut-Cushion-Diamond-1311788.asp

Size: 1.52 c
Color: F
Clarity: VS2
Table: 52.0%
Depth: 72.1%
Dimensions: 7.00 mm x 6.34 mm x 4.57 mm
Symmetry: Good
Polish: Very Good
Price: ~12k

I know the most important thing is the the visual appearance of the stone, but I’m a numbers guy and don’t really have a good eye for this kind of thing. I’ve read that the depth “should” be less than 70%, and because the most important feature for me is the overall cut and brilliance of the stone, I’m concerned about this. So I have a few questions:

*Is a depth of 72% going to adversely affect the radiance or brilliance of the diamond?
*What about “good” symmetry, versus “excellent” symmetry? How does “good” polish compare to “excellent” polish? What about variance in the thickness of the girdle? Do these things really matter once the stone is set? (In case it’s relevant, I’m purchasing a halo setting).
*I want a stone that is more square than rectangular. The above has a length/width ratio of around 1.06; the one below has a ratio of about 1.10. I was told by one of the merchants that a halo setting will help “hide” the elongation, so I have a bit more leeway here than I would with different settings. Is this true?

The other stone I’m considering is a cushion modified brilliant with some different specs:
Size: 1.52 C
Color: G
Clarity: VS2
Polish: Excellent
Symmetry: Excellent
Dimensions: 6.62 mm x 6.22 mm x 4.22 mm
Girdle: Medium to Very Thick
Price: ~13K

Note that this is 8% more expensive despite the downgrade in color. I don’t know if this is because it’s somehow more beautiful than the former, or if it’s just because it has XX in polish and symmetry, and that these are merely rarity measurements that don’t really affect the appearance of the diamond.

The stones are held by different merchants so I can’t compare them side-by-side. Any suggestions would be greatly appreciated. Thank you.

(Both stones are GIA and also boast no fluorescence or culet).