Coinbase works smoothly.  If you do not trust that they are giving a fair price as reflected on bitstamp, then transfer the coins to bitstamp and do it there manually.  From what I can tell, they are doing at least that well, matching what I see on bitstamp.  I posted earlier that it seemed I could "game" them a little, but now it seems to working more smoothly and quickly.  But they should state what price per coin you got after the transaction's done.  You have to manually do the calculation.