I think somebody mentioned it earlier that yobit is SCAM. I have no personal experience so I am following the wisdom of the crowd - I assume there are issues with the deposit/withdrawal so you cant arbitrage effectively.
I have never had any problems with withdrawal/deposit on Yobit. However, never tried out Poloniex.
Poloniex is reliable in the terms they don't run away with your money, you need to have some verification - but is currently having a bit troubles with depositing/withdrawing clams.
Deposit takes 5-10 hours to appear and withdrawals sometime end up in error state - requires contacting the support to get the coins back to the account. I got in the loop, when 3-4 following withdrawals were error.