If you only want to mine a single coin and want to get paid in that coin, go with yiimp.  If you want to try and ride the wave of multiple coins to increase your payout, zpool is the best I have found regardless of what is said here.  I am happy with the daily payouts I get with zpool and I don't have to manage it.  They at least match, but usually beat, estimated payouts for mining a single coin.  When I multi coin mine on yiimp I had to create 10+ wallets on various exchanges, then wait for enough coins to amass to be able to sell them.  Then transferring the BTC back incurred another cost.  All of this is handled in bulk by zpool which reduces cost and time.  To duplicate the number of coins available to mine and manage the exchanging of those coins to BTC, requires a significant amount of time without zpool, in order to get the same payout (in my experience).

But in the end, it's your choice what to do.  As my operation gets larger and I dedicate additional time to it, I may move to yiimp and hold coins longer to catch the rises in the market better.  But currently I don't have the time, so zpool provides a very consistent income that I don't have to manage.