Well, I already replied to your PM, but I figured I'd post here now that I'm back home and not on the phone.
The lighter (I will refer to as turquoise) one is the nicer of the two clams. If you look at the blue one (second picture), there is a flaw in the coloration on it's mantle. While I don't think that this flaw has any effect on the health of the clam and is just a naturally occurring change in pigmentation, I would NOT pay that much for a common blue maxima clam that had a flaw like that. The teal one is really nice, looks very healthy (from what you can see in a picture) and has a nice color to it, to me, that one is definitely worth the money.
I hope your tangs leave this one alone. I'm still trying to catch my beast (Sailfin Tang).
Good luck, can't wait to see pics of this!