Bill,
Just went back and took a closer look at an astrea snail in our tank and a Mexican turbo snail, comparing them to your photos above. Your first photo looks like an astrea to me, more conical, but the second photo more resembles our Mexican turbo, bigger and more rounded and curvy.

If your photos are of different snails that are both getting the job done for you, I'd invest in both as a good insurance policy. You can always pass them along in the event you have bought too many and they risk starvation after they've eaten it all (ha! should you have such a problem, right!)