It really depends on the person. Both are equally common. For example, a person working at a store would probably say two thousand five hundred just to be clear. But when talking among friends a person might say twenty five hundred because it's shorter. You can say whichever one you like best.

In the UK, we would say either 'two and a half thousand' or 'two thousand, five hundred'. The 'twenty five hundred' approach isn't really used, except sometimes for numbers between 1100 and 1900.. Eleven hundred, twelve hundred etc..

two and a half thousand dollars