no experience on manano, but I have had good sucess with fiji on my 15 and am very happy with the mix of Marshall Island and Vaniisi in my 30.
i chose to go with something other than fiji, because EVERYBODY seems to have fiji. there may be a reason, but then again, it is the most abundantly available.
If i remember correctly, the manano rock comes from somewhere near samoa and is similar to the tonga rock.