From all I've read, directly buying Bitcoin seems to have a higher likelyhood of being profitable than the best GH/s renting offers I came across, this one included.

If the exchange rate goes up, my investment directly goes up, whatever the difficulty of the moment is. If it drops, I lose a part of my investment but if I were renting some GH/s with Minershare.net, the mining would likely stop altogether if it's not profitable anymore while I still could hold my Bitcoin "coins" as long as I wish for it to recover without worrying about the difficulty increase which would make mining again with my GH/s at Minershare.net later on even less profitable compared to holding my Bitcoin money.

There is enough information at http://www.vnbitcoin.org/bitcoincalculator.php to figure that out.