You sure the betta ate him or is it hiding? They will hide especially with a betta around. As for size amano shrimp are bigger then RCS and there is a lot of debate which is better for clean up crew. I have about 7 amanos and they did one heck of job on my hair algae. As for RCS, I had some for 1 day till my bettas ate them all, but there is a reason I'm turning my 2.5g into a RCS tank. My sorority(when I had one) never harm any of my amano shrimp. If I had to choose I'd always pick amano over a cherry, just because they saved my tank from hair algae. Now I just keep them for enjoy and want a lot more.