Personally, I've always found deresa clams hardier, but that's just my personal experience and doesn't relate to either species as a whole. They are honestly both very hardy clams.
As far as looks I prefer squamosa clams, but again, that's completely subjective. Really you can't go wrong with either route.
Usually, when I come to a crossroad and can't decide which way I prefer, I let the health of the animal be the deciding factor. I.e., pick the healthiest, most colorful individual of either species that you can find. Look for animals that aren't gapping, have good consistent color throughout the entire mantle, no signs of bites, tears or any curled edges. Clams with shells bigger than 2" or so are usually hardier. Also, make sure that you inspect the underside of the animal when it's lifted for any pyramellid snails or other predators. That's a little harder to do on a squamosa because the scutes offer a great place for pests to hide, but still give it a once over. I also prefer that tridacnids are bagged underwater; not a necessity, but a personal preference that has served me well in the past...
Final point; make sure that you purchase from a store that is knowledgable about clams. Ask questions that you know the answer to and ask open ended questions like, "what do I need to know about these clams?". Even if you know the answers already, they can help gauge how much the LFS actually knows about them and how they are caring for the individuals in their systems...
Good Luck!
