Melosu has a beautiful squamosa clam, take a look at his images. I have a derasa clam which is also very hardy. I would consider where in your tank you will place the clam and also consider what fish/inverts you currently have in your tank....certain fish have been known to nip at the clam(s) mantle.
FWIW I would stay away from the maxima clams....they seem to be more delicate than others. Also, if you decide that you want a clam...realize that the smaller clams (under 2 inches) will require food....they cannot survive on the symbiotic algae (zooxanthellae) which resides in their mantles. You'll also need to maintain your calcium at higher levels to support the growth of the clam.
Good Luck!