Starting a saltwater tank

[Aikidoka]

I am thinking maybe i'll make either my 29g tank or 55g tank as a sump for my main 75g tank. which one is good, 29 or 55 for a sump?

im still searching for design of my sump. i did made a sump before for my 125g fw tank. i dont know if its the same concept.

if anyone know a layout for a good sump, please let me know.. thanks guys!