Replying to Boot /binding mounting question

when bindings are mounted do they use the shell size or mondo size or what? I always give them my skis and my boots, tell them where i want the mounted and thats all I know.
The reason im asking is my current size is 28.5 (not sure of shell size) and im looking at either full tilt hot doggers or booters (leaning toward booters for the wider last) and they have different shell sizes, 28/28.5 hot doggers are 318 shell size and the booter 27-28 shell size is 315 and 28.5-29.5 is 330