IMO the most important issue is getting healthy stock. A sick clam never seems to recover, but can look good for a long time. If you can get locally look for white growth on the shell, it's a good sign. 2nd is enough light, with the strength of LEDs now this has become less of an issues, decades ago it was issue number 1 as really only MH provided that much light for maxima or crocea, but it's pretty easy with LEDs now - I shoot for 300 PAR for those two species. Derasa and other require less, so many older references say they are easier, but IME that is more for the light issue if you don't have enough, they are all pretty tough if you get a healthy one with no pyramid snails and don't have a fish that nips at them.
So pick mostly based off light level and the eventual clam size - gigas and derasa get big, really big, but it takes a long time. Also be wary of maxima over 3", they are mostly wild caught not cultured, in fact any clam much over 3" is wild and it quite a bit harder, although not impossible.
They ship find if under 3" if healthy, in fact I would say they ship better simply due to not having all that extra weight banging them around. The whole under 3" thing needing to be fed is also a myth, I have grown out several clams that were under 1" with no phyto or anything like that, the only issue is they can be picked on or knocked around easier.
Overall I think maxima are the easiest, provided you give them enough light.
