Th darker one is much more saturated in color. More blue. As long as it is medium dark or lighter. That is the stone I would get. But not for $500.00. To much. WAY TO MUCH.
Still too much for the bluer one by $100.00 or more dollars, and way to much for the lighter gray one. I'd do some serious looking before committing. None of these stones should be more than $200.00 and really, the should each be around $100.00 a carat.
You can get two to three carat blues in both shades for $700.00. If you look.
In all sincerity.
They are lovely but I agree, very pricey particularly for that size.