I would go with the fighting conch if you are only going to get one type. They don't get as big as the queen conch's and will do more to keep the sand on the upper layer loose as they bury in it.
It's hard to say in my case. I have both in my tank and they are litlle busy bodies. Though I can say the queens grow more quickly. I have 8 in my custom 100 gallon and they get the job done!!!!
if you are refering to the tank(s) in your profile, i would definitely get fighting, the queens grow much faster and larger, upto and over 12 inches, that would be a bit large for in a 55 or aa 40 gallon tank
HTH
