Mt Gox is the main Bitcoin "Future Securitized Derivative Contract" wholesaler for most major currencies. You will generally only find wholesale (lowest buy cost) large volume prices for Bitcoin Future Derivative Contracts on that Reserve Bankstering Exchange.
Local dishonest, and risky "Reserve Bankstering Exchanges" (in your own country) trade out of commingled reserve pools of Bitcoins that they buy in bulk and mark up before sale to their local clients. This means they will easily go bankrupt in a crash since they keep everything in "their" BTC pool, unless/until extra large withdrawals occur. Otherwise they pay-out small withdrawals out of new deposits before risking the surplus of them on more backroom Bitcoin for them to play with behind clients backs. If you can figure out a way to buy BTC on Mt Gox and sell it on local Reserve Bankstering Exchanges, you got a steady spread of income, regardless of the price
An honest local "Live Trading Exchange" will keep both your funds and BTC totally segregated from their own at all times and at all stages and is much lower risk but they may still use a marked-up reserve pool of BTC of their own to conduct client trading upon, again making buying more expensive but selling better than Mt Gox prices. Technically even a totally honest and above-board exchange is at catastrophic risk of (asset) ruin from a crash, but at least they could return client cash-funds and BTC (not in Escrow) that are not theirs, afterward.
Mt Gox appears to have had problems of it's own this last week and I do not know into which group to class them as far as commingled client funds risk goes, but it's also a lot harder and more costly to arrange to get cash into them (hence, the mark-ups?). Also being the Mother of all exchanges they have no choice but to be the main "reserve", nor way to avoid "bankstering".
More about "commingling" here:
https://bitcointalksearch.org/topic/m.1845510 (Mt Gox and other withdrawal Issues)
More about "stability, fiat, derivatives, labour-exchange currency, pyramids and speculation" here:
https://bitcointalksearch.org/topic/m.1832923 (price stability vs bubbles)