It's depend on purpose and reasons on how people are choosing respective online exchanges to deal with their bitcoin activity since not all exchanges obviously have the same feature and location availability.

In my case, bitcoin trading mostly done at Bitstamp and altcoin trading purposes on Poloniex, Cryptopia and Ccex since these altcoin exchanges have the same coins listed in their exchanges.

For withdrawing purposes, look at those exchanges that can give you a less hassle way to convert your bitcoins into cash. I used our local exchanges here since using it is just one click away. Really a comfortable one.