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.
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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..